Vagary

Shapechanger. The vagary can use its action to polymorph into a beast form that resembles a gadfly (speed 5 ft. fly 40 ft.), a spider (40 ft., climb 40 ft.), or a snake (40 ft., swim 40 ft.), or back into its true form. Its statistics are the same in each form, except for the speed changes noted. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Spider Climb. The vagary can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Magic Resistance. The vagary has advantage on saving throws against spells and other magical effects.

Familiar. The vagary can serve another creature as a familiar, forming a telepathic bond with its willing master. While the two are bonded, the master can sense what the vagary senses as long as they are within 1 mile of each other. While the vagary is within 10 feet of its master, the master shares the vagary’s Magic Resistance trait. At any time and for any reason, the vagary can end its service as a familiar, ending the telepathic bond.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage, and the target must make a DC 12 Constitution saving throw. On a failed save, it takes 5 (2d4) lightning damage and, on its next turn, it must choose whether it gets a move, an action, or a bonus action; it gets only one of the three. 

Invisibility. The vagary magically turns invisible until it attacks or uses Teleport, or until its concentration ends (as if concentrating on a spell). Any equipment the vagary wears or carries is invisible with it.

Teleport (1/Day). The vagary magically teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see. If the vagary is within 5 feet of its bonded master, it can have them teleport along with it.