Strahd's undead army
After becoming the Dark Lord of Barovia, Strahd gained a level of control over the undead in his lands. While most necromancers would have to exert some effort to maintain even a few undead, Strahd does not seem to be limited in the same way. Some of Strahd's undead minions have been known to remain dormant for years while they waited for Strahd's command. He is imeadiatly aware of any new undead who enter or are created in Barovia and can exhert his control over them no matter who created them.