This is my first more detailed guide so please give feedback if you downvote!
This is story related and cannot be missed. You will get a quest called An Untimely End. Once you have learnt that that rebels are in the cistern below the temple you will firstly meet a lady in the temple who you can choose to spare or fight.
Once you make your way in to the cisterns there’s several rooms of rebels and spiders. By now the spiders should be easy to take down in a hit or two but the rebels may cause you more of a problem. On your way to one of the rooms one of your companions should warn you that there’s pressure plates on the floor. In this room either crouch and carefully walk around them towards the left far side of the room and back again or run and jump to avoid the trap shooting you. You’ll reach what seems to be the end of the cistern but find a key which fits in a double doors right near the start of the Cistern. Backtrack to this door (being careful of the pressure plates in the corresponding room once again) to go to the Lower Cistern.
In here you will have a few rooms of rebels to kill again they can be a bit of an issue depending on your gear but if you keep moving you should be fine. It’s a mix of melee and bows so keep that in mind. Eventually you’ll reach a door where a small cinematic will play revealing the person who earlier killed you Ygwulf. You have some conversations choices to make here - you can either choose to spare Ygwulf and avoid a fight or there’s a few opportunities to attack him.
If you choose to fight him he has a handful of assassins with him who I recommend taking out first just as they can be an annoyance. Once you start going to town on Ygwulf and think you have him beat he will summon more assassins including a spell caster who will take him back to full HP and continue to heal him and the other assassins so I recommend taking him out first (he’s on the stairs in the centre). Once the rest of the assassins are dealt with again you are free to mail Ygwulf once again. If you’re using a wand like I was the fireball spell was super handy should you have a grimoire that has it.
For beating him you get his armour which gives you a really nice stealth attack bonus and crouch movement speed bonus so if that’s how you’re focussing your gameplay it’s a nice piece to have.
Either way once you have either fought or spared Ygwulf this cheevo will pop!