Sisay gets +1/+1 for each color among other legendary permanents you control.
{W}{U}{B}{R}{G}: Search your library for a legendary permanent card with mana value less than Sisay's power, put that card onto the battlefield, then shuffle.
If a card in a library has {X} in its mana cost, X is considered to be 0.
14 Jun 2019
If Sisay leaves the battlefield after you've activated its last ability but before that ability resolves, use its last known existence on the battlefield to determine what cards you can find.