The Tentacles Monster is a former resident of Greenview Apartments. It appeared to be a male whose desire is supposedly to eliminate any threat to its life.
Initially, the monster takes on the appearance of a man with enlarged eyes with many tentacles protruding from its back. It uses the tentacles to both walk and attack. Its preferred method of killing is to grab and violently rip apart its victim. As Wook Pyeon noted, this creates a weakness: The Tentacless being used for one attack cannot be used for another. However, the monster is also fairly intelligent. Upon being ignited, it immediately ran into an apartment to trigger a sprinkler to douse itself. Additionally, the monster learns from its mistake. After being ignited once, it knew to catch the bottle of gas without breaking it.
The ignition also caused it to mutate. The former human's limbs mutated into spider-leg like appendages. Additionally, it can create a spike defense.
Hyun Cha and Jisu first encounter the Tentacles monster as they make their way down towards the first floor. Even with Duzik Hanh's weapons, they are unable to escape the monster and are all nearly killed. However, the commotion attracted the attention of Wook Pyeon and Hyuk Lee's rescue party. The rescue party forced Tentacles Monster to flee, but at the cost of one of the party members' life.
The Tentacles Monster is encountered later when Hyun and Wook go to rescue Dazin. Wook was nearly able to kill it, but the Tentacles Monster used the sprinkler system to save itself. It then mutated. With nothing to defend themselves, they were saved by the timely arrival of Yuri Park and Mr. Ahn, who forced it to retreat into an elevator shaft.
The Tentacles Monster remains inactive until a group of criminals take over the apartment. The commotion caused by their actions attracted its attention. It breaks into the lobby and fights everyone there. However, the criminals manage to subdue and incinerate it.