Caves are populated with different kind of creatures, some with strenghts and others with weaknesses. Surviving through those environnement allowed them to develop features, like flying, or swimming. With this, you will be able to overcome obstacles on your way. Through possession, you will be able to control and use those features to go deeper in the caves.
Before possessing, the player will have to play a minigame, to fill the possession bar in a short lapse of time.
Finding a subterreanean lake? Catch an Axolot. Stuck at deep chasm or tall underground cliffs? Possess the nearby birds. Each creature has it's own features, giving you nearly no limits, be it the air, the floor, or water.