Interesting but why not have the effect occur when the creature unburrows.
You could also have it gain 1 attack each time it resurfaces and successfully eats at least 1 creature, and have it kill any creature with HP less than or equal to its attack
This would allow the following patturn of growth:
Turn 1: 1/7
Turn 3: 2/7
Turn 5: 3/7
Turn 7: 4/7
Turn 9: 5/7
It would take 11 turns before this would be as effective as en masse upped ortys. Chaos seed / blessing might make this OP however unless it was an element othe than earth to make splasing light/entropy dificult.