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.
Strategy 1 (Uberless)
(Neo Cutter  will be the only combo cat that will be used)
Start by summoning Future cat and Eraser for meatshields, and Can Can, Manic Island and Bullet train as attackers. Level up worker cat to 3 for now. After you approach the first seal and capy, either bullet train or Can Can should finish off the seal to give you a huge load of cash. If not, just save up for A.Baha anyways.
After Cat Cannon charges, and only after it charges, send out A.Baha and Bullet train after the surge ends. Bullet train is just there so that it can either 1. Clear any peons in the way or 2. Stay in front of A.Baha so it doesn't get 1-shot by Capy. Make sure to use Cat Cannon to interrupt the Evil Emperor's attack. Continue spamming Can Can, Manic Island, and your meatshields. Make sure to time your Bullet trains and Neo Cutters to avoid hitting the surges. If all goes well, your A.Baha should kill off the first capy. If not, it should be almost dead anyway, as long as you A.Baha gets off about 3-4 hits.
Once the Old horn spawns, start adding Slapsticks to the cat spam. Obv you can summon it before if you have enough cash, but if not, only start sending it once old horn appears. If you run out of cash, prioritise Can Can over slapsticks, since you may get lucky with the double bounty and refill your cash. Avoid sending Neo cutter and bullet train unless you have sufficient cash.
The Old horn should die about 600-ish distance from your base (it seems to be consistently there for my 3 consecutively successful runs). Then summon A. Baha again and win. Hope this helps!
- 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.