I finally got to do the shadowlands campaign o gain access to the Sanctum of domination raid to get some transmog gear, i didnt realise it would be so tough but i managed to get to the last boss, i watch some youtube videos so i knew what i was doing for the fight and they say in phase 1 nothing will kill you just a bit damage, every time i try this boss i get killed in about 3 mins, i look in my combat log and it says Banshee’s Mark hits you for 1,500,000 etc, thats nothing like what i saw in the videos and it was posted only last month, am i doing something wrong or is it bugged? do all the trash need killing up to her? any help would be great thanks
In which phase does it happen?
I did Sylvanas Mythic the last 26 weeks without that kind of bug.
You just dmg her until 84% and kill the arrows in phase1.
Then you get ported and follow Jaina over the ice she conjures (kill all spawning adds) and interrupt Sylvanas on her Ruin cast in phase2.
After you damaged her to around 64%~ and ran over I think 4 bridges of ice, Jaina will make a raid teleport to phase3.
In phase3 you just damage her until 45% and the fight is over.
If you need a bit longer because you dont do enough burst dmg you will have to jump to a second platform while she is casting her Raze ability.
I think the abilitie in phase2 that you interrupt is called Ruin and the one where you have to jump in phase3 Raze, but not 100% sure atm, did the encounter mostly half zoned out. 
Edit: The adds Thrall is fighting against you can just ignore, they doesnt matter.
the very first phase this happens, around 3 mins into the fight, when was the last time you done this raid? ill try again and screenshot it, but i guess that wont help me on here as you cant post pictures 
Mhm, does it come after she gets the 99% dmg reduction and starts to cast these lines?
I was lucky and got the mount last week so I didnt do it anymore this week.
You can upload a screenshot on imgur, or any open screenshot site, and post it here as link with a dot between the http or use the <> function here when you create a post.
Alright, checked out the Dungeon Journal. I have never fought this boss, bear in mind. According to Journal, if you are not in melee and shet gets 3 stacks of Banshee’s Weapons, her ranged attack will gets you Banshee’s Marks and resets her stacks. If you are in melee, and she gets to 4 stacks, she does a melee attack that gives you Banshee’s Bane, and she resets her own stacks.
TLDR: If you get too many Banshee’s Mark, go hug her (stay in melee) to reset your stacks. In the opposite, if you have too many Banshee’s Bane stacks, go into Range and dispel yourself.
ive just tried again, and im getting 600k banshee ticks and woe from the chains are 50k but tons of them and when there are 4 lots of chains i cant make it quick enough to get rid of them
http.s://imgur.com/a/g33gRh6
Then I could only guess that the phase takes to long.
If you want you can add me and I help you with the boss, otherwise I dont know what you can do to avoid this except getting more gear so everything dies faster. 
Btag: Jay1998#2870 if you need help with the boss.
thank you for your help and offer, i dont think i can do it alone, i know im below the recommended ilvl but i managed the rest ok, if you help me that means you will be saved to the raid
Thats no problem for me, got all items from the raid already. 
thanks ill keep that in mind 