Corpus Abactor

Amorphous. The ooze can move through a space as narrow as 1 inch wide without squeezing.

Shapechanger. The ooze can use its action to polymorph into a Small or Medium humanoid it has trapped in it's demiplane, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Whenever the creature takes damage in it's stolen form, it must make a constitution saving throw (dc8+damage taken), or momentarily return to it's true form.

Demiplane. The conjurer that creates the Corpus Abactor will cast demiplane on a solid object, that the ooze will carry around with it in it's false form. This item it usually something that the false form would not look out of place carrying around, eg. a shield, or a picture frame.

The creator will force the stolen creature into the demiplane, to be kept with the ooze at all times. If the ooze moves more than 30ft from the gateway to it's demiplane, or the creature it is mimicking dies, the illusion will fail, and the ooze will return to it's true form.

True Form Only:

The Ooze makes one Pseudopod, and one Demiplane attack (if available)

Pseudopod. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage plus 7 (2d6) slashing damage.  On a hit, the target must make a DC14 strength or dexterity saving throw, or have the grappled condition.

Demiplane (Recharge 5–6). The ooze targets one creature that it has grappled. The target must make a DC 14 strength saving throw, or the ooze will begin to force the target through the door to it's demiplane. the target must repeat the saving throw at the start of their next turn. On a failure, they are forced into the ooze's demiplane. On a success, the target is released from the grapple.