Fellhammer Siege-host: The 11th-Edition Chaos Space Marines Durability Detachment

One clause, and it is aimed with real precision:

Each time a ranged attack targets a Heretic Astartes unit from your army (excluding Damned units), if the Strength characteristic of that attack is greater than the Toughness characteristic of that unit, subtract 1 from the Wound roll.

Work through what that covers. Against Toughness 4 Legionaries it applies to essentially every anti-tank and heavy weapon in the game, and to a great many mid-strength ones too. Against a Toughness 9 or 10 vehicle it applies only to genuinely heavy guns, which are exactly the ones you were worried about.

It does not apply to melee, and it does not apply to attacks whose Strength is equal to or below your Toughness, so massed bolter and small-arms fire is unaffected. That is the honest shape of the detachment: it is very strong against a focused anti-armour list and does nothing at all against a horde with rifles.

Damned units are excluded, so a cultist screen gets no protection from the rule.

Six, all 1CP, and half of them are defensive, which is consistent with the rule.

• Steadfast Determination (opponent's Shooting) — models in a targeted Heretic Astartes unit (excluding Damned) gain Feel No Pain 5+ until the end of the phase. Stacked on the detachment rule, this is a very hard unit to shift.
• Brutal Attrition (Fight) — each melee attack allocated to a targeted Heretic Astartes Infantry unit (excluding Damned) generates a D6 afterwards, up to six per attacking unit: each 4+ inflicts 1 mortal wound on the attacker.
• Siegecraft (start of opponent's Charge) — enemy units charging one unit of your choice subtract 2 from the Charge roll, not cumulative with other negative modifiers.
• Persistent Assailants (Fight) — a unit targeted this phase that has not fought re-rolls Hit rolls, and Wound rolls too if it is Below Half-strength.
• Pitiless Cannonade (Shooting) — a unit that has not shot scores Critical Hits on unmodified 5+ against Below Half-strength targets.
• Point-blank Destruction (Shooting) — a unit within Engagement Range that has not shot gains Pistol on its ranged weapons, excluding Blast weapons.

Siegecraft is the one people forget: a flat -2 to the Charge roll frequently just deletes a charge that would otherwise have landed.

• Warp Tracer (20 pts) — after the bearer has shot, one enemy unit it hit cannot have the Benefit of Cover until the end of the phase. Useful precisely because a slow, grinding list keeps shooting the same dug-in target.
• Ironbound Enmity (15 pts) — each time the bearer attacks while within range of an objective marker, +1 to the Wound roll.
• Bastion Plate (10 pts) — Chaos Lord only, excluding Jump Pack models: once per battle round, when a saving throw is failed for the bearer's unit, change that attack's Damage characteristic to 0. Ten points to cancel one big hit every round.
• Iron Artifice (10 pts) — Heretic Astartes Infantry only: the bearer's weapons gain Anti-Vehicle 4+ and Anti-Fortification 4+.

Bastion Plate is the best value here and pairs directly with the detachment's whole premise.

(Points come from the Munitorum Field Manual and move with updates; confirm in the app.)

• Legionaries — Toughness 4 infantry benefit most from a rule that punishes high-Strength shooting, which is what most anti-marine guns are.
• Havocs — a heavy weapons squad that wants to stand still, which Iron Fortitude makes survivable.
• Chaos Vindicator — the siege gun the detachment name suggests, and a Vehicle the rule protects against genuinely heavy fire; the pack gave it the Frame keyword.
• Chaos Lord — the only legal Bastion Plate bearer.
• Chaos Rhino — cover and delivery for a detachment that grinds forward rather than teleporting.

Take it when you want a slow, resilient Iron Warriors list that walks up the board and refuses to break. It is also the natural core for a gunline, because the thing a static list fears is a focused anti-tank answer, and that is exactly what the rule blunts.

Be clear-eyed about two things. First, the rule is conditional on Strength exceeding Toughness, so against volume small-arms fire it does nothing, and against melee it does nothing at all. Second, and specific to this page: this is the only Chaos Space Marines detachment the Faction Pack never mentions. Every other one is either reprinted in full or amended in the Rules Updates. That does not make it illegal, and the Munitorum Field Manual lists it normally at 2 DP with the Take and Hold disposition. But it does mean the rules text here rests on mirrors, so check the app before a tournament.

Sample gameplan: build a Toughness 4 and Toughness 5 infantry core with a Havocs squad and a Vindicator behind it. Put Bastion Plate on the Chaos Lord leading your main brick and Ironbound Enmity on a character that will stand on an objective. Advance under cover, spend Steadfast Determination when the anti-tank fire comes in, Siegecraft to break up a charge, and Brutal Attrition on the melee unit that reaches you anyway.