I did a thing, aka I repurposed some of canon future Twilight's design.
I actually had a headcanon design for the the spirit of the tree (which I drew like two times), but I decided to rewamp it for my AU.
The story is, as years went on the spirit from the Tree of Harmony's (or just 'The Spirit') form changed, probably because it became more and more powerful. It's actually unknown the whys, despite Twilight's best efforts to find it out. Or anything else really. Like why was it interested in Sandbar and his friends, why it took on Twilight's form, since when it became sentient and so on.
However, seems like The Spirit just does what it wants. Only appering when it wants to.
Even the Pillars don't how much of them planting the seed for the tree contibuted to it becoming a sentient being. Or if it has any plans.
So everything is vague, cos I'm still figuring out how I want to incorpirate the tree in my AU. I have plans, I just still thinking about the details.