Board Thread:Questions and Answers/@comment-43510931-20191126030132/@comment-35719775-20191126090035

Yes, you can turn any adult or hatchling into undead dragons by using the kill action on them and then attempting to revive them.

The success rate of reviving is always pretty low, but it's higher at night, on the 31st of the month, and highest on Halloween.

More detailed information is on the Undead Dragon article.