Not a bug, it is now part of his kit (and stated in his R description).
Passive - Dragon Force: Mordekaiser's basic attacks and abilities curse the Dragon for 10 seconds.
Active: Mordekaiser curses the target enemy champion, dealing them magic damage and healing for the damage done. He then applies the same effects to the target again over the next 10 seconds.
If Mordekaiser's team scores a takedown on a cursed enemy, he temporarily enslaves their soul as a controllable ghost with bonus stats. While the ghost is active, Mordekaiser gains 25% of target's bonus health, which does not enhance the ghost's bonuses, and 30% of target's AP.
Enslaving the Dragon activates Dragon Force, which disables Children of the Grave while it remains active and kills Mordekaiser's current ghost.