Throughout your journey in Lies of P, you find ringing phones scattered across Krat. When you pick up the phone, Arlecchino, the “King of Riddles” gives you a riddle to solve. Answer it and get a Trinity Key that opens one of five locked Trinity Rooms hidden around Krat.
There are valuable treasures inside Trinity Rooms, like cosmetic items, puppet parts, and Quartz to upgrade your P-Organ, so you should always be on the lookout. Whenever you get a Trinity Key, make sure to backtrack to a locked Trinity Door and open it to claim its loot.