Possessed by Evil (Merciless) is an Advent Stage appearing on the 11th, 12th, 25th, and 26th of every month. This is a No Continues stage. Upon completing the stage, there is a chance to unlock Stone Cat.
- Infinite Ginger Snaches spawn after 10 seconds300f, delay 20~23.33 seconds600f~700f.
- 1 Evil Emperor Cat spawns as the boss after 16.67 seconds500f.
- 3 Sir Seals spawn after 20 seconds600f, delay 26.67~30 seconds800f~900f.
- 2 Capies spawn after 13.33 seconds400f, delay of 53.33 seconds1,600f.
- 7 Sir Rels spawn after 46.67 seconds1,400f, delay 23.33~25.33 seconds700f~760f.
- 1 Oldhorn spawns after 93.33 seconds2,800f.
- 5 Sir Seals spawn after 120 seconds3,600f, delay 23.33~26.67 seconds700f~800f.
- Infinite Sir Rels spawn after 146.67 seconds4,400f, delay 30~32 seconds900f~960f.
- When the base reaches 75% HP:
- 8 Sir Rels spawn, delay 0.67~1 seconds20f~30f.
Rushers excel here when Evil Emperor Cat is alone as the enemy limit is low; here are some examples: Idi:N2, Awakened Bahamut, Immortal Yukimura, and Bullet Train Cat. It is better if the Cat has Anti-Relic/Evade Surge abilities. Although they do not work as well as the previously mentioned cats, other rusher-class cats are usable in this stage.
Long-ranged units that can outrange Evil Emperor Cat also excel here, such as Cyberpunk Cat (with the Slow Up Talent), Assassinlan Pasalan with the Slow Up Talent, and Piccolan Pasalan with Freeze Up Cat Combos. Lastly, some generally good units that can do massive damage to all enemies here are Can Can Cat (Speed Up and Money Up Talents are both recommended) and Greater Balrog Cat (Attack Up and Attack Buff Talents are recommended for higher damage). Cats in the Cradle are another viable choice if leveled high enough. Their Wave Attack can deal decent damage to the boss and the peons, not to mention being immune to freeze makes them a generally useful unit.
- The background used in this stage was added in version 10.0, but was unimplemented and had a color scheme closer to that of Saint Red Fox.
- The "Demon King" (魔王 Maō) mentioned in the Japanese name of this stage, Possessed by the Demon King, could be an allusion to Mystic Uril's description.
- Prior to the Version 10.2.1 Update, this stage had one less Capy, fewer Sir Rels and earlier enemy spawns.