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When you reach the ruins a few things can happen here depending on past decisions.
If you went to see the Unseen Elder, Dettlaff will show up and fight you immediately.
If you went to find Syanna and brought her with you, Dettlaff will try to kill her. If you gave her the Red Ribbon she’ll safely be teleported away. If you didn’t give her the ribbon she’ll die. You can then let Dettlaff go or fight him. If you let him go Ann Henrietta will throw you in prison. This leads on to the Burlap is the New Strip quest.
Assuming you choose to fight make sure you have Vampire Oil on your sword. Dettlaff is immune to status effects from bobms and is immune to Axii. Yrden will have little effect. Have Swallow Potions and possible White Raffords Decotion on hand for quick healing.
Dettlaff Stage 1 (Humanoid)
This fight is similar to your first fight against Dettlaff. He’ll charge quickly towards you and strike. Dodge out of the way and perform a quick strike. Once he’s been hit once he’ll usually dodge out of the way. Continue dodging and striking until Dettlaff transforms into his second form.
Dettlaff Stage 2 (Flying)
While Dettlaff is flying he’s invulnerable. You can hit him when he lands on the ground or when he dives at you. When he’s diving use Aard or Igni to knock him to the ground. Get in as many strikes as you can and roll away before he creates a shockwave.
His bat wave attack can be tricky to dodge. Try to get as much space between Dettlaff and yourself as possible so you can see them coming, then roll out of the way. After he’s taken enough damage you’ll move on to the third stage.
Dettlaff Stage 3 (Illusion)
For this stage you’ll be inside the illusion of Dettlaff and you need to destroy the 3 pulsing hearts while avoiding the Dettlaff clones.
The clones can hit very hard although they can be countered if you’re skilled enough. If you defeat a clone you’ll have a few seconds before another one spawns. Once the 3 hearts have been destroyed Dettlaff will be defeated.
or Burlap is the New Stripe (if you let Dettlaff get away)