TL;DR

1. What kind of Earth Elemental would be a bad enough dude to get sealed in the AT?

2. What kinds of boons or banes would this entity grant?

Hello fellow Dark Powers!

In my game I have run with the wonderful idea presented on this sub to have customized vestiges from the Amber Temple tempt my PCs. However, I'm hitting a block on one in particular.

The PC in question is an Earth Genasi Land Druid (Controller role) and I was thinking of having his vestige be an elder earth elemental. Only problem is, I know very little about elemental lore. I have no idea what the personality of an evil earth elemental could be, what it wants (aside from eating enough souls to escape and become a proper Dark Power) nor what it could have done to be deemed enough of a threat to be sealed in the AT.

For context, this PC is very bothered by how nature has been "perverted" in Barovia and wants to fix that. The player is a very gamey type who likes combat and having lots of options at his disposal. Other than that, I don't know too much about the PC, as he's a relative newcomer to the campaign.

I was thinking of having his vestige offer him the power to reform the land (in the image of what an earth elemental deems best of course). This aims to satisfy his desire of fixing the land, while mechanically creating space for lots of battlefield control goodies.

Additionally any help on mechanical boons and banes would be appreciated. Ideally they would be doled out piecemeal, starting out interesting and growing in power as he champions the vestige. Here's some options I have atm:

-If an effect would move you against your will along the ground, you may use your reaction to reduce the distance moved by up to 10 feet.

-Tremorsense

-Temporary burrow speed (upgraded to permanent eventually)

-Improvements to his go-to cantrip, Magic Stone, like giving him the option on hit to have it splinter and damage adjacent creatures, pushing the creature back, knocking them prone, etc.

-Daily free casts of flavorful spells like a Flesh to Stone, Wall of Stone, what have you

Any help is greatly appreciated