Merms spawn from Rundown Houses, which randomly appear in Marsh Biomes. A Rundown House can house up to 4 Merms, and it respawns a Merm in 4 days after it's killed. During the Day, only 1 Merm per Rundown House is normally outside (unless it gets into combat, then all other Merms come out), and when dusk comes, all Merms go outside, much like Spiders. Merms will attack if the player or any other mob gets too close to their house,or if their house is attacked or destroyed. Similar to Pigs, when they are provoked, they will attack in groups and will kite.

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Merms can be kited, very similarly to Pigs. Thanks to the hostile nature of the marsh biome, Merms are often discovered in pitched battle with Tentacles, Spiders, or both. For this reason, they make for an excellent source of Tentacle Spikes, Tentacle Spots, Monster Meat, Fish, Frog Legs, Spider Glands and Silk: Simply wait for the Merms to finish fighting, then clean up what they leave.

It is not recommended to fight several Merms at once, as they run faster than the player and will kite, like Pigs. Once the player runs some distance apart from a Merm, it will stop being aggressive and will walk back to its house.

To start a battle between Merms and Tentacles, fruits or vegetables can be used as bait for a Merm. Put at least one or two pieces on the ground near a Tentacle or more, near enough to a Rundown House. When a Merm finds the food, it will approach and eat them one by one. There is a good chance the Tentacle(s) will attack the Merm, and then more Merms will come out to fight.

They take two blasts from the Ice Staff to be frozen and just one Sleep Dart to be put under. Merms drop living Fish and Frog Legs when killed, and these remains can often be found in marshes after Merms battle with Tentacles.

Also, Merms eat fruits (like Berries), Vegetables and even Mandrakes, all either raw, cooked through fire or as a Crock Pot recipe. However, they don't eat Seeds or Honey.

With normal damage modifier characters, they take 8 hits with a Spear, 6 with a Bat Bat, 5 with a Tentacle Spike, Ham Bat and Thulecite Club, and 4 with a Dark Sword to be killed.

Merms seem to have normal walking speed, but during dusk they are faster. They are also faster than Pigs; however, they can be easily outrun as they stop when they hit allowing the player to run while they catch up. They can also be outrun if the player has a Walking Cane, or if the player uses roads and the Merms don't.