Hearts of Darkness (Merciless) is one of the Continuation Stages that the player can access after beating An Elder Secret (Merciless), the first stage in Growing Strange. Completing this stage yields an Elder Catfruit Seed, or on rare occasions, an Elder Catfruit.
- At the beginning, a Dark Otter and a Tackey will spawn. Nothing else will spawn until the enemy base takes damage.
- When the enemy base reaches 99% HP, it will spawn up to 2 Doge Darks, 2 Tackeys, 6 Shadow Boxer Ks, 5 Dark Otters and 1 Razorback. Once they are killed, nothing else will spawn.
If you plan on completing this stage whenever it comes up, you need to have Bombercat, Rodeo Cat or something similar in the lineup; that, and someone good for crowd control or damage are all that's needed specifically for this stage; as for the Tackeys, you can either lure everyone else close to your base by isolating your first bunch of units, stall them near the base while the Tackeys approach and then kill Razorback ASAP, or you can use the length of the stage to "tank" the pushback caused by the Tackeys and kill Razorback afterwards. Once Razorback is dead, you have basically won.
- Prior to version 8.0, Razorback was buffed to 150% instead of 100%, there were 11 Shadow Boxer Ks instead of 6, and the boss fight included a third Tackey that spawned 33.3 seconds after the first 2.