The Leviathan Dreadnought is available to Space Marine, Space Wolf, Blood Angel, and Dark Angel armies as a Relic Heavy Support choice.
Weighing in at 57 MBs base, it is very expensive points-wise (and gets more so if you take any of its numerous options). So, what do you get for this Land Raider(+) investment? It has WS5 BS5 S8 I4 A4 base, along with AV13/13/12 and 4HPs to make it exceptionally durable (you can even upgrade with Armoured Ceramite to make it proof against melta weapons, which is one of the few things that can easily damage it).
Wargear-wise, the Leviathan comes stock with two Siege Claws (and built-in melta guns), two Heavy Flamers on its Torso, Smoke Launchers, Frag Grenades, and Extra Armor. It also has a 4++ save from its “Reinforced Atomantic Shielding” rule, plus Move through Cover and an ability called “Crushing Charge,” which gives it TWO Hammer of Wrath hits and +1 Initiative (so I5) on the charge.
Finally, the Leviathan has multiple upgrade options, including Armoured Ceramite (as already mentioned), up to three Hunter-Killer missiles, the ability to take a Dreadnought Drop Pod as a DT, and the following weapons options:
-Leviathan Siege Claw — S: X2 AP2 Melee, Wrecker, Severing Cut (baseline weapon, Severing Cut allows for potential to inflict extra D3 wounds for each Unsaved Wound caused)
-Leviathan Siege Drill — S: X2 AP2 Melee, Wrecker, Armourbane (one MB upgrade that makes its attacks VERY potent against vehicles and fortifications)
-Leviathan Storm Cannon (costs 4 MBs to replace Siege Claw) — 24″ S7 AP3 Heavy 6, Sunder (it loses a close combat attack for each Siege Claw replaced by this, but provide strong anti-MEQ/light vehicle shooting, especially when combined with BS5)
-Cyclonic Melta Lance (costs 4 MBs to replace Siege Claw) — 18″ S9 AP1 Heavy 3, Melta (again, loses close combat attacks when equipping this, but provides potent anti-TEQ/anti-vehicle shooting)
As you can see, the Leviathan has LOTs of options and tremendous combat power potential, both in shooting and close combat, but you also pay a lot for these capabilities. In terms of making the most of the Leviathan, let’s look at the you can “force multiply” its already significant capabilities:
-MOBILITY — One of the biggest weaknesses, it can only move 6″ a turn and, while Move through Cover helps, it can potentially spend most of the game trying to come to grips with the enemy. Ways to mitigate this include taking a Dreadnought Drop Pod for it, letting it ride on a friendly Stormraven (nothing prevents this in its rules), or using friendly Psykers to “catapult” it toward the enemy with the Magnetokinesis power (from Fulmination discipline).
-PROTECTION — While the Leviathan is already very tough, it is still vulnerable to enemy MCs/GMCs, melta weapons, Necron Gauss shooting, Haywire attacks, Destroyer weapons, etc. To protect your pricy investment, there are a number of techniques, including sheltering it with a Void Shield Generator, taking the Armoured Ceramite upgrade, using terrain and Move through Cover to your advantage, using friendly Techmarines to keep it repaired, and, of course, buffs from friendly Psykers. Chief among these Psychic Blessings include Invisibility (for obvious reasons), Warpmetal Armor (making it AV14/14/13!!!), and Reforge (repairs HPs and gives IWND).
-FIREPOWER — With the right load out, the Leviathan can be an exceptional “fire base” unit, with up to 12 x S7 AP3 shots or 6 x S9 AP1 Melta shots, plus two Heavy Flamers and up to 3 Hunter-Killer Missiles (remember, Walkers can move 6″ and still shoot ALL their weapons). BS5 means these shots will reliably hit and you can combine this with Psychic powers like Prescience (for even better accuracy), Phase Form (for Ignore Cover and LOS shooting), and Null Zone (for reducing enemy Invul saves) to make them even more effective. You can also buff the Leviathan with a nearby Rhino Primaris, allowing it to Snapfire at full BS for a turn (i.e. great against Flyers, Invisible units, etc.) or with a nearby Ammo Dump from a friendly Fortification (re-roll 1s to Hit and BS5 means a 97% accuracy rate).
-CLOSE COMBAT — This is obviously where the Leviathan was built to go, as its stats and war gear all shout “CHARGE THE ENEMY AND SMASH HIS DEATHSTAR WITH YOUR UBER DREADNOUGHT”!!! With 5 x S10 AP2 attacks at I5 and swinging with WS5 on the charge, plus potential extra wounds via Severing Strike, you can easily dismantle enemy squads, vehicles, and even MCs/GMCs with the Leviathan in close combat. To enhance this even further, you can cast Prescience on the Leviathan (for Re-rolls to Hit), Unleash Rage (for 2A on the charge instead of 1A), or use Enfeeble on the enemy (i.e. reducing their T by -1, in turn allowing the Leviathan to inflict ID on anything with base T6 or lower).
As you can see, there are many ways to make the Leviathan Siege Dread even more potent and, given its high points cost, it is probably worth investing in maximizing its combat capability if you do decide to take one in your army.