FrankCheck out Wowhead's page for Pit of Saron, returning in Midnight Season 1
Trash Mobs
Garfrost Trash
Ymirjar Graveblade
Frostbane Slash - Hits the tank, applying a debuff that increases Frost damage taken by 100% for 8 seconds, can stack.
Dark Rupture - Places two AoE swirls beneath players that explode after a few seconds.
Dreadpulse Lich
Torrent of Misery - Channels into a player, stacking a debuff that hits harder and harder as stacks ramp.
Icy Blast - Hard-hitting Tank cast, can one-shot a tank, especially with Ymirjar Graveblade'sFrostbane Slash active on the tank.
Dread Pulse - At 50%, the lich will start pulsing AoE until killed.
Rotting Ghoul
Rotting Strikes - melee attacks cause a disease effect on the tank, each stack reducing Max Health by 1%.
Quarry Tormentor
Tormenting Blade
Curse of Torment
Gloombound Shadebringer
Shadow Bolt - Bolt slop.
Wrathbone Enforcer
Sunderstrike - Hits the tank with a debuff, increasing damage taken by 15%, and can stack.
Leaping Geist
Leaping Maul - Leaps to random players, will hit anyone within 3 yards. Just stay spread out.
Plungetalon Gargoyle
Plungegrip - Picks up a player, applies Stoneskin, and attempts to take them high enough to drop and kill. Burn through the shield and interrupt to release the player.
Deathwhisper Necrolyte
Necromantic Infusion - Will give a 50% haste buff to an ally. Can be purged/stolen.
Deathless Bond - Will have a few Risen Soldiers and Arcanist Cadaver summoned around that can't be killed until the Necrolyte is killed.
Arcanist Cadaver
Necromantic Infusion - Will give a 50% haste buff to an ally. Can be purged/stolen.
Netherburst - Big AoE Cast, priority interrupt.
Risen Soldier
Charging Slash - Charges a random player in the group.
Iceborn Proto-Drake
Icy Strikes - Melee attacks inflict additional frost damage.
Note: The Drakes will have a Ymirjar Graveblade mounted on them. They cast Frostbane Slash, which will increase frost damage taken by 100%.
Frost Breath - Will pick a random player to point at, spread out for the frontal so more people don't need to move.
Rimebone Coldwraith
Ice Bolt - Bolt slop.
Permeating Cold - Chooses two targets to put a Magic debuff on, will tick damage and slow the targets by 40%.
Ick and Krick Trash
Lumbering Plaguehorror
Plague Frenzy - Enrages, increasing Haste by 25%, and drops an initial Plague Spew while also dropping Blight Splatter more frequently.
Blight Splatter - Randomly throws blight on the floor, don't stand in.
Scourgelord Tyrannus Trash
While navigating the cave to Tyrannus, watch for Ice Chunks falling from the ceilingGlacieth
Cyroburst - All players need to drop a patch of ice, try and place them on a single side of the room so that you can continue to move through the tunnel unimpeded by ice patches.
Focused Guard - Shields itself from attacks, move behind it to continue DPSing.
Bosses
Forgemaster Garfrost
Garfrost will start by choosing two players to Throw Saronite at. Placement of these will be important: One the tank will destroy, leaving behind a Saronite Sludge area of denial, and the other will be used to hide from another mechanic. So keep these along the outer areas, but ensure you can still use one to hide later.
You'll need to use the Saronite to hide from Glacial Overload. Garfrost will choose one of two portals to channel into (designated by a white beam from the player to the portal), and if you break line of sight you won't take any damage.
The tank will have to destroy a Saronite ore chunk with Orebreaker otherwise they'll be stunned for 8 seconds. Garfrost will also destroy the other Saronite ore chunk with Cyrostomp after Glacial Overload. These will send out Ore Chunks when destroyed, so avoid the swirls.
Lastly, healers will need to keep the group topped up due to the Siphoning Chill rot throughout the fight. And after every Cyrostomp, two players will get the Cyroshards magic debuff. This increases Frost damage taken by 50%, and gives them a small slow effect. Dispel ASAP.
Ick and Krick
Both Ick and Krick will share health pools via Necrolink so try and position Krick on top of Ick when possible. Tanks need to be mindful of when Blight Smash is cast on them, as they'll drop Blight on the floor when hit. Ick will also Plague Expulsion while in this phase, adding more Blight to the arena.
The fight will start with a phase of Krick casting Shade Shift, which will have him teleport to a summoning circle, and also summon two additional Shades of Krick.
Shades of Krick will cast Shadowbind on a player, damaging and slowing them. While those live, Krick will cast Death Bolt, which has to be interrupted or will most likely one-shot someone. If the Shade of Kricks aren't killed off within a minute, they'll explode Shade Bomb and wipe the group. Once the two of them are killed, Krick will jump on Ick for Phase 2.
At this point, Krick will jump on top of Ick and begin directing him to attack members of the group with Get 'Em, Ick!, where Ick will Lumbering Fixate while dropping Blight all over the floor. If you're fixated, run away-- his melee can swing far. 
Otherwise, while in this phase, Krick will cast Shadow Lance for the healer to spot heal. After a few fixations, Krick will recast Shade Shift and the fight will return to Phase 1.
Scourgelord Tyrannus
On pull, Tyrannus will cast Bone Infusion which will summon several Infused Bone Piles and Bone Piles, while also giving DoTs for the healer to top off.
The fight will revolve around trying to freeze as many Infused Bone Piles (Light beams will emit from the Infused piles) as possible before Tyrannus casts Army of the Dead. When cast, regular Bone Piles will spawn Rotlings (who apply Rotting Strikes to the tank), while Infused Bone Piles will spawn Scourge Plaguespreaders, which will need to be picked up by the tank and cleaved down as quickly as possible. Focus the Scourge Plaguespreaders, interrupting their Plague Bolt and trying to limit the rot damage from their Festering Pulse.
To freeze Bone Piles, you'll use Rimefang's Rime Blast which targets two players. They have several seconds to run over to an Infused Bone Piles (and any other Bone Piles they can take with it) before the piles are frozen and they gain a Frostbite debuff.
The tank will deal with positioning the boss around these Infused Bone Piles, while also being thrown around by Scourgelord's Brand. This will knock the tank far away and apply a 200% damage taken debuff for 6 seconds. He will immediately follow up with Scourgelord's Reckoning, attempting to jump where you landed and kill the tank. Tanks need to move quickly or they'll be caught by this and will likely die.
Otherwise, throughout the fight Tyrannus will Death's Grasp around the location of players, damaging and slowing them while standing in it.
Healers will be spot-healing anyone hit by Rimefang's Frost Spit, and every so often Rimefang will cast Ice Barrage so avoid getting hit by these swirls.
