There is so much to digest with the new Traitor Codex rumors, but as I look through them, one thing that consistently jumps out to me is how powerful some of the new Relics would be when paired with a Daemon Prince (fortunately, it appears that even though certain Traitor Legions cannot take Marked Units, like Alpha Legion and Iron Warriors, they all can still take Daemon Princes with their mandatory “Daemon of X” upgrades).
We of course do not know if any of the Relics are restricted in manner that Daemon Princes cannot take (for example, it looks like some of the Iron Warrior Relics are “Warpsmith only” choices), but we know that Daemon Princes provide a very solid “base” stat line, especially with Wings to make them FMCs:
-WS9 BS5 S6 T5 W4 I8 A5 Ld9 Sv 5++
-Rules: Daemon, Fearless, VotLW, Fear, Hammer of Wrath, MtC, Relentless, Smash
-Upgrades: PML(1-3), Wings, Power Armor, Chaos Rewards, Chaos Artifacts, Daemon of Slaanesh/Khorne/Tzeentch/Nurgle(required)
Now, looking at this within the context of the new Legion Relics, here are some potentially competitive combinations between them and a Daemon Prince:
—Black Legion—(NOTE: BL Daemon Princes all get Hatred(Everyone) and Endless Hatred (Imperium), which is quite a solid buff in combat)
 Skull of Ker’ngar — Gives Eternal Warrior and Adamantium Will, both very solid buffs for any Daemon Prince, especially against opponents that can bring lots of S10 attacks against him. Combine with a Winged Nurgle DP with Power Armor and you have a VERY survivable “tank” type unit.
 Spineshiver Blade — Daemon Weapon that grants +1I to user. Upgrade a Winged Khorne DP with this Relic and on the charge you have between 8 and 12 S6 AP2 attacks at WS9/I9 (unless you roll a “1” for the Daemon Weapon), which should be devastating to enemy characters, squads, and MCs alike.
 Drakescale Plate — Gives bearer 2+ save and a 2++ against flamer weapons. Add this to a Wing DP of Tzeentch (for re-rolls of all saving throws of 1) and you have a very difficult FMC to kill without dedicated AP2/1 Skyfire shooting.
 Blade of the Hydra — S: User AP5 Daemon Weapon with Rending and Multi-Headed (every roll of 6 to Hit generates another attack until you no longer roll any 6s to Hit). Combine with a winged Khorne DP with Power Armor and you can potentially put out A TON of S6 AP2 Rending attacks each turn, depending on your “Daemon Weapon” and “Multi-Headed” rolls.
—Iron Warriors—-(NOTE: IW Daemon Princes get FNP(6+), which is a nice little boost to durability)
 Axe of the Forge Master — S:+1 AP2 Armourbane, Master-Crafted, Unwieldy. Since Monstrous Creatures ignore Unwieldy, a Slaanesh DP with Wings, Power Armor, and this Relic can be a premier Tank Hunter, using its 12″ Move + D6+3″ Run to get across the table turn 1, then charge a vehicle turn 2 with 6 x S7 AP2 Armourbane attacks (with MCed for a re-roll in there too).
 Fleshmetal Exoskeleton — Another Relic that grants a 2+ save to its bearer, but also IWND, which combines quite well with a the 4W on a Daemon Prince. Upgrade a PML(3) Nurgle Winged DP with this Relic and roll to get Fleshy Abundance power on the new Nurgle discipline (i.e. adds back D3 wounds lost earlier in the battle) and you have an exceptionally difficult to wound, and keep wounded, FMC to threaten you opponent with.
—Night Lords—(NOTE: NL Daemon Princes get free Stealth, which gives them a 3+ save when Jinking as FMCs)
 Scourging Chains — For only 2 MBs, you get Shred on your Melee attacks, which is an amazing upgrade on a DP that already has 6 x S6 AP2 attacks on the charge naturally. Almost a mandatory upgrade on a Night Lords DP that is not taking a different Relic already, I think.
 Stormbolt Plate — Gives bearer a 2+ armor save and +1 to its Cover save. Combine this with a Winged Tzeentch DP and you have a ridiculously durable FMC… 2+ armor, 2++ cover save when jinking (Jink + Stealth + Bonus to cover save from Relic), and re-rolls of all 1 to Save (so a 97% chance of saving wounds against any shooting attacks that are not both AP2/1 AND Ignore Cover!).
 Curze’s Orb — Re-roll all 1s for Run, to Hit, and to Wound rolls. Again, for only 4 MBs, this is a great buy on a Daemon Prince, as it makes mobility and hitting power that much more reliable for a very reasonable points cost.
—Word Bearers—(NOTE: WB Daemon Princes harness WCs on a 3+ for any Malefic Powers)
 Crown of the Blasphemer — Gives a 4++ and Adamantium Will. Combine this with a PML(3) Tzeentch Daemon Prince rolling hard on the Malefic table for Cursed Earth (i.e. +1 to Daemon Invuls and no Scatter for DSing Daemon units within 12″) and you can have a 3++ save, with re-rolls of 1s, to make quite a tough target to engage (on top of this, the DP can summon more Daemons to support your army and/or use Malefic witchfires to soften up nearby enemy units).
 Cursed Crozius — Basically a Power Maul that gives the bearer PE: Imperium, this not only bumps a Daemon Prince’s attacks up to S8 AP2 (or S9 AP2 on the charge for a Khorne DP), but also makes them much more dangerous against any Imperium forces, which accounts for a lot of the enemy armies you are likely to face (and all for 7 MBs, which is pretty reasonable as far as Relics go).
—World Eaters—(NOTE: Of course, WE Daemon Princes MUST have Daemon of Khorne upgrade)
 Talisman of Burning Blood — While this is most likely to be used on an Independent Character in order to boost an entire attached squad with its benefits (i.e. +3″ to all move, run, and charge ranges for a unit), allowing a Winged DP to move 15″ and charge 2D6+3″ means you have a very solid chance of a first turn charge with that FMC, especially if he comes from the World Eater’s special detachment that gives a free, pre-game 2D6″ move (not a Scout move, so you can still charge turn 1 after doing this) and free re-rolls of failed charges.
 Berserker’s Glaive — A two-handed Daemon Weapon that grants the bearer +2S, IWND, and FNP (at the cost of losing Independent Character, which DPs do not have anyway), this weapon means a Khorne Winged Daemon Prince with Power Armor can generate between 8-12 S9 AP2 attacks on the charge, all the while protected by 3+/5++, T5, FNP, and IWND abilities.
—Thousand Sons—(NOTE: TS Daemon Princes with a Blessing Cast on them improve to a 4++ save; also, they must be Daemons of Tzeentch, of course)
 Seer’s Bane — A Daemon Weapon with Force that uses the bearer’s Ld for Strength and the opponent’s Ld for Toughness. If you successfully cast Force on the Tzeentch DP, then not only does it improve to a 4++ save, but you also gain Instant Death for this Daemon Weapon. Given that a Daemon Prince has Ld9, you will wound most enemies on a 3+ or even 2+, but against enemies with Ld9 you will wound on a 4+ and against opponents with Ld 10, on a 5+. Obviously, since the majority of enemy close combat beasts are Ld 10, this would not necessarily seem the best weapon for a DP in challenge (i.e. wounding on 5+s), but given that all wounds are AP2, occur at WS9/I8, and have Instant Death with Force activated, I think this is actually better than one might expect on first thought. This is especially good against very high Toughness opponents, like Wraithknights, which DPs would be wounding on 6+s anyways, since this gives you a real shot at killing them quickly (i.e. Instant Death eliminates their FNP rolls and removes D3 wounds per Unsaved Wound). NOTE: If you can get multiple Heldrakes from a Heldrake Terror Pack within 12″ of your opponent, or you can charge them with two units from a Raptor Talon, then you can lower their Ld significantly to make this an even more powerful weapon.
 Staff of Arcane Compulsion — This might actually be an even better option for a Thousand Sons Daemon Prince than Seer’s Bane. Basically a Force Maul (S: +2 AP4, Force, Concussive) with an ability that causes enemy charging the bearer to suffer a -2 penalty to their charge range), this Relic gives a Tzeentch DP 6 x S8 AP2 attacks on the charge with Concussive and Instant Death, meaning that is very strong against squads of infantry, light-medium vehicles, and even tough ICs/MCs (thanks to Instant Death from Force).
—Death Guard—(NOTE: DG Daemon Princes gain FNP, but also have -1I, so are I7 base, and must be Daemons of Nurgle)
 Pandemic Staff — Both a Ranged (Template S:1 AP5 Poisoned(2+)) and a Melee (S:+2 AP4. Concussive, Poisoned(2+)) weapon, for the costs of 6MBs this gives a Death Guard DP some solid versatility, with both anti-horde shooting and S8 AP2 Concussive/Poisoned attacks in combat (don’t forget, if your “natural” strength is higher than your opponent’s toughness when attack with a poisoned weapon, then you get to re-roll all failed to Wound Saves, so against T7 and below enemies, a DP equipped with this weapon basically also gets Shred for free).
 Plaguebringer — A Daemon Weapon with S: User AP3 and Poisoned (4+), this combines quite well with a Death Guard/Nurgle DP. Daemon Weapon gives between 2-6 bonus attacks (all S6 AP2, thanks to the DP being a Monstrous Creature) on top of the base 6A on the charge, unless you roll a 1 for the Daemon Weapon, and poison allows for re-rolls to Wound against any opponents T5 and below, so a Daemon Prince equipped with this Relic makes for an excellent “squad killer” type HQ choice.
—Emperor’s Children—(Note: EC Daemon Princes all gain FNP(6+) and get one “free” attack before dying if slain in the Assault Phase)
 Intoxicating Elixir — For the price of 5 MBs, you get D3 rolls on the Combat Drugs chart, which means the Emperor’s Children DP can boost between 1-3 of hit stats in the following categories: +1WS, +1BS, +1 I, +1 A, +1S, or +1T. While WS, BS, and I are not that great (you already have super-high stats in those categories), buffs to number of attacks, strength, and/or toughness before the game even begins can take a Daemon Prince from formidable to outright devastating. If the DP is part of the Emperor’s Children special detachment, then he would actually get D3+1 rolls on the Combat Drugs table.
 Endless Grin — Another very cheap Relic (2 MBs), this causes enemy units within 6″ of the bearer to have -1Ld. Combined with other Ld reduction shenanigans, or even with Psychic Shriek on the Daemon Prince himself, and you have a very solid force de-multiplier for very little points costs, especially when combined with the mobility of a Swooping FMC like a Winged DP.
As you can see, there are A LOT of very competitive options here for Daemon Princes (and I did not even hit on all the Relics; there are still plenty of other combinations out there that might work well). Of course, we have to wait and see if there are restrictions on certain Relics (i.e. like “Sorcerer only,” or “Chaos Lord only,” or “cannot be taken by Daemon Princes,” but judging by the precedence set by the old CSM Codex and Wrath of Magnus, I expect only a few Relics to be restricted in this way. Hopefully this is true and we will see many powerful new Daemon Prince builds coming out in the near future (right now, I think the single most competitive one will be a Tzeentch Night Lords DP with Wings and Stormbolt Plate… being able to get a 2+ save, 2++ Jink save, and re-roll all saving throws of 1 makes for an almost invincible Monstrous Creature that already has very good mobility and close combat threat capacity!).