Berserk. Whenever the golem starts its turn with 40 hit points or fewer, roll a d6. On a 6, the golem goes berserk. On each of its turns while berserk, the golem attacks the nearest creature it can see. If no creature is near enough to move to and attack, the golem attacks an object, with preference for an object smaller than itself. Once the golem goes berserk, it continues to do so until it is destroyed or regains all its hit points.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

Magic Weapons. The golem's weapon attacks are magical.

Siege Monster (Ram Head only). The golem deals double damage to objects and structures.

Charge (Ram Head only). If the golem moves at least 20 feet straight toward a target and then hits it with a gore attack on the same turn, the target takes an extra 16 (3d10) piercing damage and the target is grappled (escape DC16) as it becomes impaled on the golem's ram and if the it isn't already grappling a creature. Until this grapple ends, the target is restrained

Fetid Aura (Gas Head only). A 10-foot radius of spores and yellow fungal fog billows from the vents on the Gas Head Skaab's helm. The fog and spores spread around corners. When a creature enters the spell's area for the first time on a turn or starts its turn there, that creature must make a DC16 Constitution saving throw. The creature takes 9 (2d8) poison damage on a failed save and must roll on the Short Term Madness table, or half as much damage on a successful one, suffering no effects of madness. Once a creature has suffered the effects of madness from this trait once, it immune to its effects for 24 hours.

A creature holding its breathe while in the fetid aura to counteract the spores effect that attacks, takes damage or takes some other action that takes physical exertion must make a Constitution check against a DC16 or half of any damage taken. When a creature fails this equal to 1 plus its Constitution modifier, it instantly gasps for air and makes the Constitution save against the effects of the fog at disadvantage and can no longer hold their breath until they finish a long rest.

Greatsword (All except Headless). Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 19 (2d12 + 6) slashing damage.

Hurl (Ram Head and Gas Head only). Ranged Weapon Attack: +9 to hit, range 60/240 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage.

Gore (Ram Head only). Melee Weapon Attack: +9 to hit, reach 10ft., one target. Hit: 45 (3d10 + 6) piercing damage.

Call For Assistance (Spell Sink only): After sapping enough energy from those casting spells around them, the teleportation ring mounted on the skaab's shoulders whirs up and conjures forth a demon from the Abyss as per the Summon Greater Demon spell. Once the skaab has used this action, the teleportation ring becomes inert and cannot use it again.

Multiattack (Headless only). When a creature that isn't an undead or a construct comes within melee range, the golem makes three attacks. 

Spell Sink (Spellsink only): The skaab attempts to interrupt the first creature that it can see within 30 feet in the process of casting a spell of it and must make a Constitution check. The DC equals 13 + the spell's level. On a success, the creature's spell fails and has no effect. Once the skaab has successfully countered four levels of spells in this way, it can use its Summon Greater Demon spells action.