Sleight of Hand: 15

And after a moment, the door clicks. The door slowly opens and inside the room, you can hear chanting.

If nothing came out to attack the party Sylvester says “Didn’t that door lead to another place? Oh well. On ward, I’ll slip in to the shadows Kastor or Brozznan maybe you can be a distraction so the other and I can get a better vantage to disrupt what ever is happening over there.”

Sylvester will sneak in to the room and see what going on.

Stealth: 11

Perception: 12

"I think we can all just try to proceed as quietly and carefully as possible. No point having a distraction to attract their attention if we can slip in unnoticed."

Stealth: 5

Perception: 21

Edit: I guess I'm the distraction then, with those rolls! lol

The room is dimly lit by a single magical flame, levitating in the air close to the ceiling. And although Kastor trips upon entry, swinging the door open and making it hit the wall, the chanting does not stop. As the group comes closer, the chanting can be made out, "Free this one of death's holds, let this one master the mortal realm. Break time's grasp upon his vessel, no limit upon his power, and to finally reach divinity. Absolve the curse of mortality." The room holds two hallways, each one leading into a brighter room, each one with a shadow of a robed figure.

Kastor whispers to the others, "Sorry to be reckless, but I think this ritual sounds like it needs to be stopped as quickly as possible."

With that, he casts Ice Knife at one of the mages.

Attack: 10 Damage: 9

Then (hit or miss) the target and any others within 5ft must make a DEX13 save or take 7 Cold damage. 

(My last post had rolls keep changing when I tried to edit it to add initiative, and then to explain them changing. So I’ll just reroll everything to try and keep it fair)

Sylvester is going to attack the mage that was hit with the Ice Knife.

initiative: 16

Attack: 26

Damage: 5

Sneak Attack: 5

Bonus attack: 14

damage: 4

(Both make a Wis save. DC of 12. If fail, then you'll have to redirect or lose your attack. Both of the acolytes are protected by sanctuary so even if Kastor chooses the other target upon fail, he'll also have to roll for another Wis save. For Sylvester, you'll have to use your dash to get to either of them and if fail, then you'll lose your attack.)

Sylvester will use his bonus action to dash and forgo my bonus action attack

Wis Saving Throw: 8

WIS saving throw: 25

So choose the other target and another WIS save? 17 

Dex Save: 15

With a strange compulsion upon the group, they find themselves somewhat unable to harm the mages. As Kastor aims for one, he shoots the other one. He shoots his knife of ice but misses, with even the mage still avoiding the explosion of ice. Sylvester runs to the Kastor's first target, but the compulsion forces him to stab the ground. 

The mages spare no attention to the party as they continue chanting. Sylvester can see that before the mages is a ceremonial bowl, filled with blood as it swirls and starts to shine.

Sylvester will throw a dagger at the bowl of blood trying to break it or spill it.

attack: 22
damage: 6

(Just to clarify what we see - the room we came into, is it empty? Is that where the bowl is? And there are two rooms just off that first room, each with a chanting mage in it?) 

(Each mage has a bowl before them. The room before is empty. If you need an example, check the spoiler.)

 Sylvester breaks the bowl, sending the blood all over the pedestal upon. The mage scowls but does not stop chanting. The blood still shimmers.

Kastor will run to the other mage, pick up the bowl in front of them, and throw the bowl to the floor. 

The bowl shatters, sending blood all over the floor, some hitting the mage and Kastor as well. "We offer the blood of the faithful, take the power they hold and let this vessel succeed in divinity." The mages' chant still goes and seems to be almost done.

Frustrated that the ritual still seems to be continuing, despite their best efforts to interrupt it - with the mages apparently committed to continue the ritual until either it is complete or they are dead, and doubting his ability to kill the mage in time, Kastor suddenly has one idea that might just work.  He takes out the flask of Holy Water, and pours the contents into the pool of blood on the floor, and kicks it around with his feet, hoping to thoroughly contaminate the blood being used in this ritual with the holiest thing he has.  Whilst he doesn't know the specific nature of the ritual, he's pretty sure it's necromancy and hopes the holy water now spread through the blood will nullify the magic.

(Actually, might Kastor know anything about the specifics of the ritual?  Being proficient in both Arcana and Religion?)

Sylvester in a last ditch effort will try to attack the closest mage to him not wanting to feel completely useless seeing that Kastor had a spark of inspiration.

Wis ST: 11

Attack: 17

Damage:5

The holy water mixes with the blood, and the shimmering stops. The mage curses as he turns his attention to Kastor. "The intruders tainted my blood brother, finish the ritual!" He draws a club.

Mage 2 Int: 7

Sylvester tries for the mage once more, only to be stopped by the compulsion once more. He stabs the ground as the mage before him replies to to his comrade, "Of course!" He bites his thumb and draws blood. He thrusts his hand into the blood upon the floor. "Let this vessel break his mortal bondage and ascend from this plane. To lead and teach others of the power within!" 

(Sanctuary is broken upon Mage 2 but still active upon Mage 1.)