It's late here, but wanted to give you at least a basic answer to your question. Solak, Merethiel and Erethdor were the first characters I wrote and even designed, so their stories aren't fleshed out fully within the game and lore so far... I hope to be able to expand more upon them in the future
As someone else mentioned here, Solak is of anima mundi but way less of a threat than say Telos or Vorago just based on his mindset and desire to protect. Powerful, but likely won't ever use it.
Merethiel and Erethdor however... (Pinch of salt with what I had/have planned for their story) These two tapped into darker and forbidden elven magics from the grimoire that Erethdor stole. A type of magic so incredibly dark and comes at a massive cost to the user and the world, by this I mean, to use the magic, the user needs to tap into the earth or nature around them.
Erethdor being the powerhungry elf he is, really took this to another level with his attempt to use the heart of the grove's lifesystem as a sort of 'supercharged attunment' for the darker magics, but was thwarted by his siblings, Merethiel, Saeldor and Faerdhinen, who I hope you get to meet in the future.
Merethiel however, realises what she did when they first tapped into the magic and refuses to use it ever again.
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