What if I forget about using my UNITE ability?
All cards with the UNITE ability are optional affects that only take place when announced. If a player forgets to activate a UNITE ability then the effect does not take place.
Note: In friendlier games it is often good sportsmanship to allow players to go back and use a UNITE ability even if it was initially missed, but that is up to the player(s). In tournament/league play this would not be allowed.