The undead psion would be a very rare zombie variant capable of operating psi gear.
They don't despawn.
They may spawn with some simple spells and some armor pieces.
They have inventory space for all armor pieces, a CAD, a sword, a vector ruler and a slot for any block.
They pick up anything that fits into one of their slots. If the slot is full, they swap its contents with the item on ground after 20 seconds.
They don't do anything with the vector ruler, their CAD can.
They attack with their sword.
When they aren't attacking they continuously cast the selected spell in their CAD.
They take breaks between two casts.
They remove conjured lights in a 3x3 area around them because undead mobs like the dark.
When a spell they cast executes trick: conjure block, the block will have a dark tint and only lets a small fraction of light pass through, again because undead mobs don't like light. They might have a light sensor psimetal helmet with a spell that conjures a block above their head, thus preventing them from burning.
Trick: conjure light works differently for them too. It places a "black light" that spawns dark particles around it, prevents undead mobs from burning in sunlight, gives blindness to entities too close to it and maybe make nearby blocks have less light.

Why would this mob be a good addition?

• More challenge for singleplayer survival
  With spells good enough even hard to kill mobs aren't really challenging. Ender dragon? Explosive projectiles kill it pretty fast. There's nothing (at least in vanilla) that comes even close to your powers. This mob would be an interesting challenge.
• A quirky solution to automating things - and we even avoided boring boxes!
  You could capture one, give it a CAD with a spell you want it to cast, put it in a cage in the right place and you have an autocaster. It's far from being a boring box and also quirky to set up. Why? Because it'll try to kill you the entire time. You need to be very tricky to put one in a cage in the right place, you need to avoid it's attacks before you trap it and make it pick up a CAD without a spell it could use to escape or harm you and lastly you need to protect it from sunlight, iron golems and such.
  Why swap equipment after 20 seconds?
  Because this way you can give it a CAD you want and swap the CAD you gave it any time as long as you've successfully trapped it in a small cage. The 20 seconds delay is to prevent things like throw an empty CAD at it and it's instantly disarmed, being unable to cast anything useful. This way you need to trap it first.