Caustic Sentinel

Unyielding. This construct will always succeed against spells or attacks meant to destroy its body after death. 

Disorienting Embrace. If the Caustic Sentinel ends its turn grappling a creature other than a construct or undead, that creature must make a DC 16 Constitution save or be poisoned until the start of Caustic Sentinel’s next turn. While poisoned in this manner, the creature has disadvantage on all saving throws.

Desperate Embrace. The construct makes grapple checks and its grapple attack with advantage. 

Bloom. When Caustic Sentinel takes any amount of fire damage, it immediately dies in a fiery detonation. When Caustic Sentinel detonates, all creature within 10 feet of it take 2d6 fire damage and must succeed on a DC 18 Strength save or be knocked prone. 

Grapple. +10 to hit. The construct stumbles forwards, trying to grapple a target. Due to the highly damaged body the Caustic Sentinel exhibits, the grappled creature can contest the caustic sentinel's grapple with a bonus action. If this interaction fails, the creature can use their action as normal. Upon succeeding, the sentinel is knocked prone.