The Cursed Laboratory is a spawn location for Missing Ninja . It's free and can be unlocked in 2 ways; Either run up through the Cursed Caves or get rid of all the money on your account and die, then you will see the option to spawn in the cursed laboratory.
After running through the Cursed Cave you will reach the Cursed Laboratory Entrance. In the entrance of the laboratory there's a sealed box containing the Koitaru boss battle. You need between 25 (50% spawn chance) and 50 (full unseal) DNA to summon the boss.
Inside the Cursed Laboratory you find many different NPCs. Such as Suragami where you can set your spawn point. There's also Zabuto who can help your craft the Poison-Laced kunai for a set of ingrediants. This Laboratory where the cursed hosts are created, so here you can find "mutating hosts".