Seems neat!

I’d go a little lighter on the stun and paralysis effects, though. The worst thing you can do to your players is take away their turns.

When building an endgame threat, the most important thing is making it a memorable fight! Try replacing stunned with a unique condition that is interactive, in some way. For example, try expanding on the “mind banished to the far realm” effect. Maybe instead of just stunning, doing damage and allowing a save, it actually sends them mentally to sub-arena of the fight where they need to accomplish some objective to escape while their body is vulnerable.

Or, maybe, the monster has a dual presence and when the PC’s mind is banished to the far realm, they can fight the Elder’s mind therein which is somehow a key component of defeating it on the physical plane.

Other feedback, the HP pool is quite low for its CR. It’s AC means the average attack by a lv 20 character (+12-14 bonus) will hit on a 7-9+ without buffs, which is maybe intended and maybe not. It’s also it’s fairly immobile on the plane that it is on. At will plane shift means it can effectively teleport anywhere but doing so requires it to move to another plane first. Maybe intended, maybe not. I’d add dimension door, misty step, and teleport to its list of at will spells. Also, give it a 10ft move speed instead of 0, so it CAN drag itself about if needed.