I used Mongo quite a bit last week because it was Wild Plains week. My observations of its possible effects:
“Something random happens.”
- grants (Magic + 10) to all (4) stats of a random ally
- transforms into a random troop (rarity restrictions?)
- deals (Magic + 2) heavy splash damage to a random enemy
- inflicts a random bad status to all enemies
- explodes many (?) gems
- devours a random enemy