Without any other qualifications, the sheer toothy-and-tentacly-ness would be enough to get the Star Spawn Emissary pretty high on my list of favorite monsters. Hailing from Van Richten’s Guide to Ravenloft, they’re also pretty nasty in a fight. They generally start out as a Lesser Star Spawn Emissary. These beings are able to change shape and blend in with their surroundings. A CR 19 with legendary resistances and legendary actions, they are a hard combat encounter for a party of 4 level 13 PCs.
Sadly, for the PCs, the terror does not end when this creature hits 0 hit points. They return as a Greater Star Spawn Emissary. This form of the creature is a CR 21. It loses all the abilities that made it a capable infiltrator, relacing shape change with the ability to spew acidic bile.
The Star Spawn Emissary is Tough
If this Star Spawn Emissary survives its encounter, it can return to its Lesser form after completing a long rest. Both forms of the creatures deal psychic damage with ranged attacks and piercing and acid damage with their melee attacks. With their legendary actions they can teleport and attack for a single action.
These things are a nasty one-two punch for a party. Presumably, they have to meet this creature and get it to reveal itself in some way before the fight begins. Be sure to consider what resources the party will use in getting to reveal itself, and then the XP budget for both the lesser and the greater form. These creatures are highly mobile as well, and so consider planning for them to flee before the Greater form is destroyed. Then the party has time pressure to find them before they can regain their full power.