Vampiric Hamster Tornado

Double initiative. Each of the tornado's attacks occurs on a separate initiative, both made at a +7 check.

There's a lot. Has resistance to spells that treat it as a single entity.

Open floor plan. There's a 25% chance a ranged attack will pass through harmlessly.

Super swift. Does not incur attacks of opportunity.

AC = DC. The AC of the creature increases as it loses health:

• 250-275 HP: 14 AC
• 200-249 HP: 16 AC
• 100-199 HP: 18 AC
• 1-99 HP: 20 AC

Lots of Pain. The tornado makes two attacks on separate initiative rolls of +7.

Thrash. Melee Weapon Attack: +12 to hit, reach 5 ft., 1 target. Hit: 3d8 + 12) [bludgeoning] damage. The target is immediately [grappled]. Tornado can grapple as many as two medium or one large creature simultaneously. While grappled, at the start of a creature's turn, is takes 2d6 [piercing] damage. Requires a Athletics check, DC 17, to escape as an action. 

Spin Cycle. The tornado uses its movement. All targets that it passes within 5 ft. of during the movement need to make a DEX save DC 17 or take 8d6 [bludgeoning] damage. The tornado heals itself for half of that damage. On a successful save, creature takes half damage. If at least one creature hit on the attack fails the save, the tornado heals for half of 8d6, otherwise a quarter if they all save. If anyone misses the save by more than 7 points (i.e., DEX save of 9 or lower), they lose 1d10+5 HP and are thrown (1d4)*5 feet away from the hamster. The lose HP can be regained on a long rest, or a character can spend 1 extra hit dice on a short rest to regain 5 HP in this way at a time.

Pa-tooey. After taking damage, the tornado can elect to eject any grappled creature at the attacking creature. The target must pass a DC 17 DEX check or be knocked 15 feet away from the tornado, take 3d6 [bludgeoning] damage and be knocked prone. On a successful save, no damage and not prone. The ejected creature must make a separate saving throw of the same difficulty or take 3d6 [bludgeoning] damage and half on a successful save.