The Skylanders who are associated with the Undead element are mostly skeletal, ghouls, or creatures who have been corrupted by the element. They use the power of the undead and darkness to aid them in battle. They are strong against Life, but are weak against Magic characters in the Battle Arena. The colors for this element are black and white.
When opening an Undead Gate in Spyro's Adventure, the Undead Skylander calls upon living bones to form a bridge. Undead-based Skylanders stand on a pedestal that resembles a white skull when they're in their toy form. The white skull on the pedestal glows in the dark.
In Giants, spikey bones rise from the ground and act as gates to Undead elemental zones.
Malefor is the Dragon King of the Undead and was the one who corrupted Cynder into doing his evil bidding. He also attempted to capture the elven sorceress Skylander, Hex, but was soon confronted and defeated by her.