No. 3 - Sins of the Father
It’s hard to find anybody who thinks that the fight against Vanstrom Klause is not a fight which makes Sins of the Father worthy of being a Grandmaster quest.
The overarching belief was that Jagex wanted to reserve that status for the next quest in the Vampyre/Myreque series, especially since the player still hasn’t defeated Lowerniel Drakan himself.
And it was recently confirmed that that quest, titled The Blood Moon Rises, is slated to be released in early 2026, and it is indeed set to feature a fight against Lowerniel Drakan.
The first phase of the Vanstrom Klause fight features three special attacks, two of which give you very little time to react. You must kill the Bloodveld he spawns with Ranged to avoid it approaching you and dealing massive damage. When he spawns the blood orb, you can use it to damage him by pulling him into it; otherwise, it will heal him.
And then there is the darkness attack. If not reacted to by turning away from the vampyre pretty much instantly, you will take massive damage.
Once you get his health to zero, he heals for 200 life points and goes into a de facto enrage phase, in which your reflexes are also tested with consistent lightning strikes. You have very little time to react when the affected tiles show the dark circle, and it may be hard to determine if you are standing on an affected tile as it is.
Damien Leucurte is also no pushover as a minor antagonist of the quest. He switches between Ranged and Magic attacks and can also poison you, and you must stamp out the fires he starts around the room in order to be able to do consistent damage.