He is immune to all debuffs and CC except for CT down (doesn't help much).
He is immune to surety.
All his physical attacks have a chance to instant kill.
He has around 30k HP.
Boss Skillset
Normal Attack = Relatively high damage normal attack, does very little to backline.
Brain Death Nightmare = EXTREME Dark physical Nuke on whole team.
Obsidian Grave = Summons two Undead Mages with ~1.5k HP each that do Prayer Offering (instant kill move with 1 turn charge)
Scythe of Nightmares = Powerful Dark Row physical that has chance to fear
Trial of Silence = Montino basically
Omega Crest = Mazelos basically
Sanctuary Blast = Lazelos basically
Soul Perfection = Single target -20 Fortitude, -30 SP and MP
Silent Deathbringer = EXTREME Dark physical single target damage
General Fight Characteristics
The fight always starts out as an ambush.
He will always start with Brain Death Nightmare.
He will cast Obsidian Grave out of turn at a certain HP level. Happens twice. Once at around 7k-8k HP and once at 25k-27k HP.
He will cast Brain Death Nightmare out of turn as a certain HP Level. Happens once at around 15k-16k HP.
He can move twice per turn relatively often, especially if your team's overall ASPD is low or drops due to teammates dying.
Suggested Setup
As much light gear as possible. Farm Den of Light for every character.
Run at least 2 Priests and 3 very high damage DPS (Fighters preferred).
Stack as much Divine Power as possible on the priests. You will spend most of the time casting Madios.
If possible, put as much Instant Kill Resistance on the MC + key units on your team. However, even with high Instant Kill Resistance, he still can proc it quite frequently.
Try not to bring Light Units, as all his attacks are Dark element and do heavy damage.
Make MC Dark element + give him a lot of Instant Kill Resistance + Get Deep Look Back and Reversal Resonance.
Try to have over 500 total ASPD on the team to prevent him from going twice every turn.
Strategy (Assuming Level 60 Cap)
Pray you only have a maximum of 1-2 characters that die at the start.
Keep track of damage, so you can slow down DPS at the ~15k HP mark for most of your team to defend. Knight's Defense works too if you can time it well.
git gud at timing resurrect
You will want to just focus entirely on DPSing Morgus down.
Kill the mages summoned by Obsidian Grave in a timely manner.