Nice idea!
My suggestions:
- when it dies, it gives just one layer of Bone Wall (instead of a 7-pack). (Note that it will increase an existing BW stack by 3)
- reduce the cost a lot. Make it cost one or two quantums, max.
- keep the current effect as it is, but reduce its cost to one quantum as well.
This way, we have a nice original little fella, that will be easy to cast, that will be a good thing to sacrifice, and that will have an interesting way to recycle your pillars during the mid game. Slightly strong, but not OP.