Levistus's_Leviathan
5e Freelancer
I'd also like to add that the Oblex was created by a child that used their wish from the Make-A-Wish Foundation to meet with Wizards of the Coast.Ironically, my main answer here is a monster new to 5e. The Oblex.
I included an "elder oblex" in my DW game. It had found (or, more likely, been given) a psi-tech means of extending its lifespan past the point at which a normal oblex would have to divide or else die from containing too many memories. This allowed it to control a significant number of people remotely, without needing a direct slime connection, and it had one specific person who had (by all indications willingly) given herself over as a vessel for the elder oblex to use so it could interact with the outside world while it's actual body was stuck inside the machine.
The players defeated it by using a combination of a portable (but single use) antimagic field device and a "thermite spike" that could pierce through the armored shell of its life-support machine. Of course, they had to fight through some of its nastier servants to get close enough to deploy the spike, and had had to do some reconnaissance in order to know what the hell they were up against in the first place, but it was definitely a fun challenge, a nice simple conflict after some previous heady stuff.
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