With the new and (greatly improved) Draigo in the most recent GK Codex and the new Mephiston as well, I see the possibility of taking the two greatest BAMFs of the entire Imperium and having them tag-team up on anything and everything!
If Draigo joins Mephiston, the entire unit uses the highest Toughness value, so they both tank wounds at T5. If there is a low AP hits coming, Mephiston can Look out Sir them off to Draigo, who uses his 3++ and EW to absorb them. If you are getting a high volume of Torrent of Fire shooting (like say from Tau Missileside Teams), then use Look Out Sir to hand them off to Mephiston if Lord Kaldor starts to run low on wounds. Mephiston is PML(3) and Draigo has The Aegis, so that makes their combined unit very good at Deny the Witch rolls on witchfires/maledictions directed at them. Finally, the unit is Fearless, so it will never run (and it can score, like all units now in 7th edition).
Mephiston should generate his powers from Biomancy of course. Anything he rolls can be a great boon their unit:
1 – Iron Arm — Makes Mephiston T8 (so the whole unit has a majority T8), S8 (S10 with Hammerhand from Draigo), and his attacks become AP2 from Smash USR.
2 – Warp Speed — Mephiston gains +3A/I, turning him into even more of a blender than he already is.
3 – Enfeeble — Lowers the ID threshold for a Target unit and makes it even harder for that unit to wound the already T5 (or T8, with Iron Arm) Draigo/Mephiston team
4 – Hemorrhage — Alright power, combined with Draigo’s Purge Soul, this two man unit can put out some decent shooting damage in the Psychic Phase… one of the least useful options Mephiston could roll.
5 – Life Leach — Great little power, not so much for its damage output (decent against armored troops), but because it can help Mephiston and/or Draigo regenerate lost wounds, making them even more difficult to kill….. talk about frustration for your opponent……… IT……….WILL……NOT………DIE!!! Aaaarrggghhh!!!
6 – Endurance — Ah, the ultimate power ; With this on, Mephiston gains EW (so now you have EIGHT T5 wounds with 2+ saves and EW) and both he and Draigo get FNP(4+), basically doubling their survivability against almost all attacks!
As you can see, you really can’t go wrong with rolling on Biomancy three times with Mephiston (Oh, and he gets the Primaris, Smite, as an extra little bonus).
Now, what does good old Kaldor bring to the duo? As already mentioned, guaranteed Hammerhand, meaning that Draigo will be fighting at S9 AP2 and Mephiston will be hitting at S7 AP3, or S8 AP3 when charging due to Furious Charge (both have force weapons for ID). Additionally, Draigo gives you anti-Daemon abilities (Banishment and PE: Daemons) in case they are fighting a Daemon Prince and the bonus of Psykout grenades when fighting Psykers.
Finally, Draigo has guaranteed Gates of Infinity… that means that this little 2 man unit can pop smoke any turn and reappear anywhere on the board (with DS scatter factored in, of course), giving them ridiculous mobility, though they can’t charge that turn. Turn 1 they can “bounce” from your DZ to the enemy DZ in a single move and be completely in your opponent’s face before he has the chance to react at all. Also, since they are just a two man unit, the chances of scattering and having a Mishap are small…. even if they do, 5/6 times they will either go into Ongoing Reserves (to come in next turn) or be placed by your opponent which, fortunately, you can use Gates of Infinity to overcome the next turn (and that is in the worst case scenario). In the best case scenario, you either don’t scatter at all (lucky roll) or you use BA Scouts with Locator Beacons or Stormravens with Locator Beacons to ensure an accurate “bounce” through the warp.
If using Gates of Infinity is still too risky for your tastes (given that this unit costs about the same as two Land Raiders), then you can load them up in a friendly transport (Stormraven, Landraider, allied Chimera, etc), or have them footslog across the board.
If you choose the latter, I highly recommend you invest in a cheap Fortification (like Wall of Martyrs Bunker) with an Escape Hatch… deploy Draigo/Mephsiton in the Bunker before the game begins, then turn 1 disembark them out of the Escape Hatch (which can be anywhere within 12″ of its parent building)… this in essence gives you an 18″ move turn 1, plus whatever D6″ run Draigo and Mephiston can muster…. basically, you are in the enemy DZ and ready to charge him turn 2 without using a transport, Gates of Infinity, or any other crazy combination.
Aside from the very real close combat threat that these two monsters bring to the fight (10 S8/9 attacks on the charge with Hammerhand on, all at WS6/7 and I5/6), the psychological threat is probably the biggest advantage this combo has. Anyone who knows even a little about 40K understand how potent even one of these Heroes is and will likely use most or all of his combat power trying to bring them down. Fortunately, they can really take some damage.
Against BS4 Bolter fire, it would take 27 shots to even cause a single unsaved wound to them (54 Bolter shots if Endurance is cast on them). If Mephiston get Iron Arm off, then they T8 and bolters can’t even hurt them!
Against BS4 Plasma fire (with Draigo tanking the hits), it would take 5-6 shots to cause a single unsaved wound, going up to 10-12 shots if they have Endurance for FNP(4+) and up to 27 Plasma shots to cause a SINGLE wound if they have both Endurance and Iron Arm activated.
Against BS5 Railgun shooting (from Longstrike) it would take an average of 4 shots to generate a single unsaved wound, which Draigo can absorb with EW.
Against BS4 Centurion Grav Cannon/Amp shooting (with Draigo tanking) it would take 4-5 shots to cause a single unsaved wound (that goes up to about 9 shots for an unsaved wound if they have Endurance cast)
As you can see, these two can absorb tremendous punishment away from the rest of your army or, if your opponent ignores them, they can rip up his forces quite handily. Very expensive, definitely a Deathstar, but also very potent and potentially quite fun to play!