The Wraiths, otherwise known as the Elemental Wraiths were godlike beings originally from the realm of Argent D'Nur
The Night Sentinels were formed to protect and worship them, as the peons of D'Nur feared and revered them.
They empowered the Night Sentinels, making them nearly invincible against the demonic forces of Hell.
A Night Sentinel that lost his son in the battle that was raging at the time approached a Hell Priest, Deag Grav and a pact was made, if the Betrayer would lead them to the Wraiths, he would get his son back.
The Betrayer was tricked, and after he lead Deag Grav and his Cabal to the Wraiths as they slept, the Wraiths were cursed and subjugated, and were used to create the Well. The Betrayer got his son back, but in demonic form, as the Icon of Sin (The Wraiths III)
Their general appearance prior to their sighting in Doom 2016 is unknown, as it has been shown that exposure to Hell's energy in copious amounts will drastically change the visual and internal structure of beings.
For the next three ages, until the fourth age the Wraiths were under Hell's corruption, their elemental energy being siphoned into powering The Well, giving demonkind unlimited energy to use in their conquests.
Following the invasion of the UAC facility on Mars, and the subsequent invasion/ the re-awakening of the Doom Slayer, the Wraiths were freed with The Crucible after the Slayer retrieved it from the Necropolis following the defeat of the Hell Guards.
In the ruins of Argent D'Nur, the Wraiths were strung to sacrifical altars that siphoned energy from their bodies in arena like structures, and enemies wishing to free them would be required to clear the arena of Demons sent to battle them. What is interesting is that before more are spawned, only a few imps and a singular Hell Knight reside in each arena, but the fight will spawn demons ranging from the Hell Razer to the Baron of Hell (Excluding the Summoner and the Possessed/Unwilling)
To free the Wraiths, the player jumps up on the chest cavity and activates the Crucible prongs, stabbing it and twisting into a molten ball in the cavity, creating a large argent explosion and removing one of the beams from the ball in the middle of the area.
Once all three Wraiths have been freed, another area will open where the player can fight the Spider Mastermind.
The Wraiths had great energy within them, clearly shown as they could have powered the Well for so long (Centuries or thousands of years, although the time frame is unknown) and it is presumed had they not been freed they would have been powering demonkind for many years more to come.