The Lich's Skeletal Guards appear briefly in "Mortal Folly," attacking Finn and Jake when they enter The Lich's lair. They serve The Lich to attack and stall intruders so he may drink from his Well of Power uninterrupted.
The Skeletal Guards look very similar to the skeletons that attack Finn and Jake in "Death in Bloom," but the Skeletal Guards consist of skeletons of various sizes. They are likely the remains of pre-war people who had made their final resting grounds in the old subway station which contains The Lich's lair or those The Lich had previously killed.
They include the two Boyfriend & Girlfriend Skeletons, and one skeleton that has red hair and a red eyeball hanging from its eye socket. This one is also much larger than the other Skeletal Guards and has the remains of a cloak on its back. The Guard startles Finn and Jake when it rises from underneath them. However, it is destroyed by Billy's gauntlet, which makes a hole in the ground, leading to The Lich's inner sanctum.
Unlike the skeletons from "Death in Bloom," double-killing them seems to have no effect. The skeletons Finn smites with The Gauntlet of the Hero quickly rise again to continue their onslaught. They also appear to lack the ability to talk, but this could be because they are fully under the control of The Lich.