Blightsteel Colossus

Adamantine Plating. Any critical hit against the colossus becomes a normal hit.

Defiling Aura. Ordinary vegetation within 10 feet of the colossus withers and dies. Within this area, Plant creatures and magical plants have vulnerability to necrotic damage and disadvantage on saving throws, and attack rolls against plants have advantage.

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

Indestructible. If the colossus is subjected to an effect that would kill it instantaneously without dealing damage, that effect is instead negated.

Mythic (Recharges after a Short or Long Rest). If the colossus would be reduced to 0 hit points, its current hit point total instead resets to 180 hit points. Additionally, the colossus can now use its Trample bonus action during the next hour. Award a party an additional 20,000 XP (40,000 XP total) for defeating the colossus after its Mythic trait activates.

Rejuvenation. Unless its lifeless remains are reduced to ash, disintegrated, otherwise utterly destroyed, or placed under an effect that would prevent them from becoming Undead (such as a ceremony, gentle repose, or hallow spell), the destroyed colossus re-forms in 1d4 hours, regaining all its hits points and appearing in the unoccupied space nearest to the largest piece of its remains.

Siege Monster. The colossus deals double damage to objects and structures.

Unusual Nature. The colossus doesn’t require air, food, drink, or sleep.

Multiattack. The colossus uses Infect twice.

Slam. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 33 (4d12 + 7) bludgeoning damage plus 24 (7d6) necrotic damage, and if the target is a Medium or Small creature, it is grappled (escape DC 17). Until this grapple ends, the target is restrained, and the colossus can't grapple another target.

Infect. The colossus uses Slam. On a hit, the target also must make a DC 20 Constitution saving throw if it is a creature. On a failed save, the target's hit point maximum is reduced by an amount equal to the necrotic damage taken, the target's Strength score is reduced by 1d4, and the creature is infected with the disease Glistening Oil. The target dies if this reduces its Strength or hit point maximum to 0. Any effect that removes a disease allows a creature's hit point maximum to return to normal. Otherwise, the reductions last until the target finishes a short rest. On a successful save, the creature can’t be infected by Glistening Oil for 24 hours.

While infected by Glistening Oil, the target is poisoned. While poisoned in this way, the creature can’t be reinfected, and it must repeat the saving throw at the end of every 24 hours. On a failure, the creature gains one level of exhaustion as the infected creature’s body is slowly taken over by Glistening Oil. When the creature reaches six levels of exhaustion, the creature doesn’t die and is instead compleated and all levels of exhaustion are removed.

Trample (Mythic Only). The colossus moves up to its speed in a straight line, ignoring difficult terrain. Any object in its path takes 55 (10d10) bludgeoning damage if it isn’t being worn or carried. During this move, it can enter Huge or smaller creatures’ spaces. A creature whose space the colossus enters must make a DC 21 Dexterity saving throw. On a successful save, the creature is pushed to the nearest space out of the colossus’ path. On a failed save, the creature falls prone and takes 35 (10d6) bludgeoning damage.