He puts his mark on magical creatures for various reasons. He placed his mark on Countess Cassandra and Chimera when they became his allies, but he also used it on other people to make them become his servers, like he did to the mermaids of Andros, which he also turned into mer-monsters. His mark also appeared on Sky's shoulder and on Radius's neck when they were under the control of Diaspro and Cassandra respectively.