Initially, Antamsunchies start out at 10, but their number can go up to 20 as Antasma creates more of them to increase their force.
Antasmunchies do not attack on their own when Antasma is active, but he will eat them to increase his attack power. There is one attack where he gathers them into a ring. Mario has to avoid getting caught outside the ring in this Chase Attack lest he will be blown up, and he must also steer clear of the ring's edge. When Antasma closes his arms, Mario must use his jump nigh immediately as the transformed Antasmunchies close in on their target. If the ring "bites" him, Mario will be blown up and possibly sent to a nightmare. Also, Antasma will get hurt if Mario makes his escape from the ring.
Antasma can also confiscate Dreamy Luigi, rendering him useless until Mario destroys the Antasmunchie containing him. If Antasma eats him with an Antasmunchie, he will gain an extremely high power boost.
Antasma can also rest when he is low on stamina. When Antasma does this, he becomes dormant and heals 200 Heart Points every turn, while the Antasmunchies enter their aggressive active form. During this phase, Mario will get hurt if he stomps them, but he can use his hammer to dodge their head spike while damaging them. Antasmunchies can also charge at Mario from all for sides, but Mario can use his hammer to block their rampage. Once they are all destroyed, Antasma will become irate and wake up so the Antasmunchies can be resurrected.