I once made a Mimic that, though it was originally in the shape of a sword, made a deal with one of my players in exchange for being able to constantly drink his character’s blood. When the player first got the sword he quickly realized it was basically stuck in his hand and overnight, the sword expanded adding a gauntlet over his hand, where the Mimic then sunk its teeth in waking him up but practically ensuring that if he didn’t cooperate he’s loose a hand. So they struck a deal and began to work together. The mimic eventually grew strong enough to take on the shape of full plate armor in addition to the sword by consuming the character’s blood, granted the character lost more permanent HP but gained slightly more temporary HP. Eventually it was revealed that under the armor, which he couldn’t take off, the mimic had begun slowly consuming his flesh starting from where it had first made the gauntlet all those adventures ago. Basically the player was missing his arm up to the elbow but while using the mimic basically still had an arm. So, he made the tough decision to let the mimic eat him if it meant achieving his goal of revenge from his backstory. Long story short by campaign’s end he was just a skeleton, heart, brain, and eyes inside the mimic which now had total control of his body(Though I still let the player do choose his actions etc based on the Mimics goals). The other players mind you had no idea he was basically dead and so when the Mimic Knight turned on them and they killed him they were horrified to find the only their former ally’s dying remains within.
Easily my favorite use of a mimic.