Kellan Joins Up is a rare Legendary Enchantment card from Outlaws of Thunder Junction (OTJ), collector number 354. Current market price: $0.41 (foil $0.43), TCGplayer market prices via Scryfall.
When Kellan Joins Up enters, you may exile a nonland card with mana value 3 or less from your hand. If you do, it becomes plotted. (You may cast it as a sorcery on a later turn without paying its mana cost.) Whenever a legendary creature you control enters, put a +1/+1 counter on each creature you control.