Gathering of the Elves

At the beginning of your upkeep, put a muster counter on Gathering of the Elves.
{g}, {t}: Put a muster counter on Gathering of the Elves.
{2}{g}, {t}: Amass X elves, where X is the number of muster counters on Gathering of the Elves.
(To amass X elves, put X +1/+1 counters on target army creature you control. If you control no armies, create a 0/0 Elf Army token first.)