Assimilation

Sticky black sludge rises from the ground and engulfs 1 creature that is Huge or smaller within range. The target must make a Constitution saving throw. On a failed save, the target takes 8d12+30 necrotic damage and for the next minute, its speed cannot exceed 10 feet. On a successful save, the target takes half as much damage and its speed is not limited.
A creature reduced to 0 hit points by this spell is dissolved by the sludge and rises as an Oblex at the start of your next turn. This Oblex rolls its own initiative and acts on its own turn. You have control over it for 1 minute, after which it becomes hostile to all creatures except you. An Oblex created in this manner does not have the Innate Spellcasting feature unless it was created from a creature capable of casting spells. The size of the Oblex spawned depends on the size of the creature it was created from.

Small and below: Oblex Spawn

Medium: Adult Oblex

Large and above: Elder Oblex