Winged skeletal constructs originally servants of the Nehekharan god of death, but turned to servants of Nagash. They were among the first models to start veering away from Warhammer’s traditional design philosophy of “historical fantasy” and looking much more fantasy than history, though some resemblances can be connected to angelic beings with a few visual cues from gargoyles.
After Nagash's second defeat he sent a plague to wipe out the defenders of Nehekhara, and with 90% of the living populace dead he prepared for another military invasion with even more corpses. The god Ptra, god of immortality, was enraged by the disruption of his cycle and sent his servants, fleshy beings with angel wings carrying blades forged in a star (we don't have an image of what they looked like alive, just a description of them rotting) called Hammurai to destroy him. For the Moses amount of time (40 days/nights) they battled, but in the morning of the 41st day Nagash somehow killed all of them and caused the non-bone organic material to rot off them and raised them as undead with all their former consciousness but somehow the loyalty to Ptra replaced with loyalty to Nagash (somehow). He felt they were superior to the Vampires who had just previously failed to defend him against the Nehekharan forces thanks to their arrogance and backstabbing, since the Hammurai (now called Morghasts) were already a fighting unit of servants. The polarity of the magic within them reversed from undead-destroying light to undead-raising death, and as they cut down the few remaining humans in their way they left behind even more soldiers to lead. As Nagash prepared the Great Ritual to kill everything in Nehekhara and raise it in undeath he was assassinated by the last living Nehekharan King Alcadizaar using the dreaded Skaven blade known as the Fellblade. The Morghasts had been split between forces in the north ready to sweep over the rest of the world and the ones of the south who defended their master, who attempted to carry out his will. But the partially completed Great Ritual had succeeded in killing everything in Nehekhara and raising it from the dead, but failed at commanding their spirits; as a result EVERY SINGLE NEHEKHARAN WHO EVER LIVED arose at once, and quickly put down Nagash's loyal forces (before going to unending war with each other). Unable to overpower Nagash's enchantments, the Morghasts were trapped in tombs in a resting state (which explains why we never saw them in Warhammer prior to End Times).
Necromancers would discover the remains over the years, but thanks to the creation of the Vortex by the High Elves there was not enough magic left in the world to keep them animated. Even when Nagash returned he himself was unable to awake them.
Nagash was only able to raise them in the beginning of End Times as a side effect of ripping Death magic from the Vortex, and later his consumption of the entire pantheon of Nehekharan gods including Ptra. They went about carrying out his will of turning the whole Warhammer world into a land of the dead, until the forces of Chaos (of all things) put a stop it. They also joined Nagash and the other Incarnates for the final battle against Archaon, with Nagash putting most of them under Arkhan's command to hold the line while the Incarnates tried to stop Archaon creating a third Chaos rift.
Age of Sigmar
The Morghasts were the first bone constructs of Nagash and prototypes for the Ossiarch Bonereapers (their models directly influencing how the OBR were designed). They serve both as elite shock troops for the various legions of Nagash and as the eyes and ears of the Great Necromancer, with the Archai also serving as bodyguards for Nagash's generals. They will report any sign of treachery or failure they see in the undead commanders and will not hesitate to put them down at the slightest command from Nagash.
Morghasts in AoS have individual names and are also capable of speech (at least the Archai variant, and only in the novels so far), but it is unknown if this applied to the WBF variants too.
Both could be taken by either the Vampire Counts or the Undead Legion combined Vampire Counts/Tomb Kings army. Tomb Kings had no access to them, since the army was technically destroyed and made into the Legion at the start of the event while the Vampire Counts were folded in over time later.
- Harbingers: Special category, 80 point Monstrous Infantry with Hover and Terror. Statline is not bad, ringing in with 5WS, three S5 Killing Blow Attacks, and 4 Wounds at Toughness 5. Their Initiative isn't too bad either, at 4. It has two Hand Weapons and Heavy Armor, so it's geared pretty well. This alone however isn't why you bring them when you could get a Sphinx. They have the special rule "Heralds of the Accursed One" which causes all friendly troops within 12 inches to take 1 less Wound from Combat Resolution damage. IT STACKS, and they're 2 or more to a Unit.
- Archai: Rare category, for 90 points you got a solid statline (S5 T5 W5 A3), 4+ and halberds. Rest of rules as Morghast Harbinger, including anti-crumbling aura. All in all, nice support unit to provide resilience to your unstable units, capable of surviving in melee, if need be. Take this if you want to save your Special points but want that buffing.
Age of Sigmar
Usable in Legions of Nagash and Ossiarch Bonereapers, now only the latter, both variants are monstrous combatants, with 6 wounds, 4+ armor, and a bravery debuff on top of it.
- Harbingers: The shock trooper with 3D6 charge designed for chopping through hordes like a flying blender.
- Archai: The bodyguard who can shrug off mortal wounds and made for fighting heroes and other elite units.