Monstrosity of the Lake is a rare Legendary Creature — Kraken card from Tales of Middle-earth Commander (LTC), collector number 105. Current market price: $2.58, TCGplayer market prices via Scryfall.
When Monstrosity of the Lake enters, you may pay {5}. If you do, tap all creatures your opponents control, then put a stun counter on each of those creatures. (If a permanent with a stun counter would become untapped, remove one from it instead.) Islandcycling {2} ({2}, Discard this card: Search your library for an Island card, reveal it, put it into your hand, then shuffle.)