The Council of Blood is an encounter which has the raid fighting against three bosses with separate health pools. When one boss dies, all others heal to full and the remaining bosses gain additional abilities! The raid will need to get on their dancing shoes, as the bosses force the entire raid to take part in the Party Foul each time one of them reach 50% health. Those with poor dancing skills will lose out on a big haste buff! 

Council of Blood Quick Tips in Castle Nathria

• Focus on killing one boss at a time. Recommended order: 1st: Baroness Frieda 2nd: Castellan Niklaus 3rd: Lord Stavros

• When a boss reaches 50%, get to your dance position and then move to the your marker over and over again until the dance ends.

• Stay within the dance floor area whilst fighting the bosses to avoid the Party Foul damage. This doesn't apply during the Danse Macabre.

Baroness Frieda

• Interrupt her Dreadbolt Volley casts. Dispel the debuffs applied if a cast is accidentally let through.

• Spot-heal players targeted by the Drain Essence.

• 1 Boss Dead: Spread out when Prideful Eruption is cast.

• 2 Bosses Dead: Taunt half-way through the Soul Spikes to have each tank hit by two spikes each.

Castellan Niklaus

• Taunt swap after two Duelist's Riposte casts

• Nuke down the Dutiful Attendant add which is making the lowest health boss immune!

• 1 Boss Dead: CC and kill the Dredger Servant which deal low damage to random people.

• 2 Bosses Dead: Tank the Veteran Stoneguard and kill it as soon as possible.

Lord Stavros

• Face away from other players when tanking this boss due to the Evasive Lunge. Taunt swap after 2 casts.

• When paired up with Dark Recital, find your partner and keep moving from the patches you spawn!

• 1 Boss Dead: Move away from the Waltz of Blood dancers.

• 2 Bosses Dead: Find the single Dancing Fools that can be killed. Kill them and stand near their corpse for a safe spot!

A full guide can be found on Wowhead: https://www.wowhead.com/guides/the-council-of-blood-castle-nathria-raid-strategy-guide