It is highly recommended groups bring a healer or at the very least a good amount of off-healing, even for endgame players.
The Scorion Broodlord is a unique Daedric Monstrosity similar to Velidreth in its attack patterns although there are notably three new mechanics that the players are introduced to:
1) A mass of deadly AoEs that spawn out from the Broodlord that need to be sidestepped or healed through.
2) A massive debuff DoT on the person holding the taunt which requires a large amount of healing.
3) Feeding on the obelisks located around the boss room that needs to be prevented by destroying the obelisk that the boss is feeding on.
Like Velidreth from Cradle of Shadows, the Scorion Broodlord does attempt to feed on players but this can be interrupted unlike Velidreth's feeding. Every 20% Health starting at 80% of its Max Health, the Scorion Broodlord will summon four adds that require attention from the tank and healer.
These adds are an Armored Daedroth, Xivilai Ravager, Xivilai Shockslayer and a Flame Colossus. Each of the four adds deal a ridiculous amount of damage through their respective mechanics which require a large amount of healing.
As the tank and healer handle the adds, the two DPS in group should go destroy the obelisk that the boss is feeding on. Repeat until boss is dead.
Cryonin Artellian is quite an interesting boss. As a battlemage-type boss, the boss summons mechanics that the players get hints of throughout the dungeon up until the boss encounter. The red waves must be sidestepped or avoided by all group members.
The boss also does a devastating heavy attack on whoever is holding taunt.
When the boss is channeling it receives a large shield but will stop channeling once it tries to one-shot group members by sending out large red skull projectiles that can be easily roll dodged given enough distance.
Accompanying this boss at the start of the fight and after every red skull phase are two Storm Atronachs that hit quite hard. Once these two Storm Atronachs are dead, two Storm AoEs will spawn on two players who can actually situate these AoEs before they actually spawn. Bolt Wyrms also spawn that can snare players until they die which is tied in with the red waves you need to avoid.
The last boss can be quite difficult without a dedicated healer or at the very least good group healing. The Magma Incarnate will apply a bleed DoT on application of its Light Attacks on the tank while occasionally heavy attacking that needs to be Blocked or Roll Dodged.
The boss also summons Scamps that hit quite hard with their projectiles and Heat Waves, these need to be cleaved down whenever possible.
The boss also sends out rail-line AoEs that target three out of the four players who must not overlap with one another once these rail-lines shoot out from the boss. Once the rail-line shoots out per player, wherever that player was, the line leaves a large AoE that's quite painful. It also leaves a harsh DoT debuff on the players who were targeted that must be healed through. Purge does not work in cleansing these DoTs.
The boss will start summoning fire cyclones that move around static positions and also in a rotating wall that moves around the room. The only gap in this fire wall is between the machine in the center of the room and the wall itself so group positioning needs to be on point.
Sometimes the machine that birthed the Magma Incarnate will also do a massive AoE explosion that fills up nearly the entire room. While you certainly can run out of the AoE to the edge of the room, this may complicate your positioning as a group if the wall of fire cyclones are about hit you. This wall cannot be Roll Dodged.
Furthermore, the Magma Incarnate slam his swords into the ground which require at least one to two players to step into his golden AoE to prevent the group from taking damage/dying.
At 65% Health, the Magma Incarnate will become invulnerable while pulsing out damage. This will lead to the Portal Phase where the entire group must go into a portal and end up in a randomized environment to destroy an Obelisk that can spew out mechanics or summon adds (giant spiders, spinning flame lines, rotating ice cyclones, etc) before the Scorion Collector spider completely feeds on it making the main boss fight harder.
Once the Portal Phase is finished, the group must come back out or will eventually be teleported out to the main boss fight where the wall of fire cyclones starts rotating in conjunction with the Scorion Collector spawning as well in addition to the Scamps. It's highly advised that the Scorion Collector be the DPS priority as the boss will easily get cleaved down to its next health increment.
The Portal Phase mechanic occurs again at 35% Health thereafter the Scorion Collector will not immediately spawn but will spawn sometime into the resuming fight.
The Purgator is the first secret boss you can discover and fight by first unlocking the Secret Boss room in The Dread Cellar by going to the area before the first boss encounter.
Two blue Daedric flames must be picked up by two players and must be dumped into two braziers as indicated by the map below before the flames' durations run out on either player.
Once done, you can access the Mysterious Corridor through a hatch where you must find two Defense Matrix Shards (Orange Crystals) as indicated by the map below.
After this you must dump two blue Daedric fires into the braziers by the large orange crystal in the Secret Bosses room.
After defeating the Purgator, you gain access to the Purgator for the main boss fights while also gaining the Purgator's Accession buff:
The Undertaker is the second secret boss you can discover and fight before the second boss encounter and you do so by locating and then carrying three blue Daedric fires into three braziers as indicated by the map below.
Once done, you can access the Mysterious Corridor through a hatch where you must find three Defense Matrix Shards (Blue Crystals) as indicated by the map below.
After this you must dump two blue Daedric fires into the braziers by the large blue crystal in the Secret Bosses room.
After defeating the Undertaker, you gain access to the Undertaker in addition to the Purgator (if you'd found/fought the Purgator as well) for the main boss fights while also gaining the Undertaker's Accession buff:
The Grim Warden is the last secret boss you can discover and fight before the last boss encounter and you do so by locating and then carrying FOUR blue Daedric fires into FOUR braziers as indicated by the map below.
Once done, you can access the Mysterious Corridor through a hatch where you must find and interact with four Defense Matrix Shards (White Crystals) as indicated by the map below.
After this you must dump two blue Daedric fires into the braziers by the large white crystal in the Secret Bosses room.
After defeating the Grim Warden, you gain access to the Grim Warden in addition to the two other secret bosses (if you'd found/fought the previous 2 as well) for the main boss fights while also gaining the Grim Warden's Accession buff: