Skelmis attacking Call.
Skelmis is an Air Elemental. It attacked Callum Hunt in The Bronze Key. Skelmis was send by Alex Strike to try to kill Call.
When Aaron Stewart and Callum Hunt use chaos magic, they summon a chaos Elemental. The chaos elemental fights off Skelmis, and when Masters Rufus and Milagros come in, Rufus uses air magic to disperse Skelmis.
He also may or may not be one of the chaos elementals brought back by Alex from chaos. However, he was at the rift to the void.