Beyond Divinity Edit
A Death Knight is a demon who serves a master or a mistress. Death Knights are mostly used as elite guard. In Beyond Divinity, Samuel uses Death Knights as his personal guard, and when he is summoned away he orders his elite guard to capture the hero and his Death Knight companion.
"as ordered your excellency, they shall await your return, from inside a cell"
"I am a Death Knight, a servant of utter chaos. I served Isolde loyally for many years, but now I am one of the elite guards under Samuel's command."
Divinity: Original Sin Edit
As you proceed, you find out that Leandra creates them with Source magic, Tenebrium, and her blood. She used the Source magic and these creatures to destroy portal to the First Garden and harm Zandalor during the quest Follow the Wizard.
You must use Death Knight Bane first to get rid of their invulnerability. You have to bypass or flee from Death Knights until you learn the spell. To make the spellbook, you need Vial Of Leandra's Blood and Leandra's Spell.