Very cool that you're involving the community in the design of your legendary summoner! I really like the backstory of a Dark Eternal that defects and "goes white".

I think the CONVERT ability is a perfect fit for your backstory.

Here's another idea for an ability:
CONFUSION - The summoner uses his magic to throw out a burst of blinding light, dazzling the enemy team and causing them to stumble about in disorder. At the start of each turn, there is a 10% chance for this ability to trigger. When triggered, the card order of the opposing team is reversed (i.e. last card becomes first, 2nd to last card becomes second, etc)