When entering his cottage during daytime, he will likely be hunched over the Arcane Enchanter in the corner, with his back to the door. With a high enough Sneak skill or potion, it may be possible to perform an initial sneak attack.
Drelas will often retreat upstairs and attempt to heal himself, so a good strategy is to use a Magicka Poison during the initial attack.
Once killed, Drelas does not respawn.
If the Harmony spell is used on him (he is too powerful for low-level Calm spells), he will speak trespassing NPC lines such as, "You're not supposed to be in here."
Although he is a Dunmer, he uses the same voice as many male Altmer.