Marco's great-grandfather Diego Avila was a member of the Men of Letters who fought in World War I. His experiences in World War I drove Diego insane, which Marco describes as "Jim Jones style" to the point that all he could see was blood and violence.
In 1925, Diego performed a a summoning spell that led to the release of a goddess named Yokoth who slaughtered Diego and some of his comrades. After Yokoth was bound, the survivors managed to close the rift Diego opened and bind Yokoth before she could succeed.
In the aftermath, the Men of Letters disavowed the entire incident, banned all the survivors and sealed the chapter house. Unable to kill Yokoth, the survivors and their descendants, led by Diego's family, worked to keep Yokoth sealed up. This continued onto Marco and his sister who continued to monitor the bunker where Yokoth was trapped with Marco getting a job at diner not too far from the area.
In The Thing, Marco is at his job having a laugh with a coworker before he notices Sam Winchester and Dean Winchester as they bring in Yokoth (masquerading as Sandy Porter). He is speechless at her release and calls his sister before getting the orders from the brothers and places an unknown substance in it.
Ophelia arrived with more descendants of the men of letters, with Marco slipping out after the power goes off. As Sam consumed the food he drugged, they take the unconscious man away while leaving their comrades to guard the area and Yokoth.
In the forest, Marco and his sister Ophelia Avila question a waking Sam on his letting a monster out. They explained the other men of letters' bunker, as he reveals his family is tied to the organization a discovery that causes them to reveal the same goes for them. They explained their great grandfather went mad after he became disgusted by the world after seeing nothing but chaos. They continue by revealing a rift was opened and a god called Yokoth emerged from it and slaughtered some of the people who summoned it before the rift was closed before its mate could escape. They tell Sam it was bound in Sandy's body and that descendants of survivors such as them were guarding with Marco telling Sam they have been unable to kill it so they resorted to starving it before he sarcastically gives Sam thanks for freeing it and tells him of the danger of when Yokoth eats.
Seeing they were on the same side, they release Sam and return to the diner where they find the bodies of their fellows drained by Yokoth. As Sam looks for his brother, Marco revealed he must have been taken for Yokoth to breed.
They then realized Yokoth returned to the bunker and headed there. Once there, they found Yokoth conducting a ritual to bring Glythur into the world while using Dean as a host. They attacked her which kept her distracted for Dean to free his right hand and snatch the Seal of Solomon from Yokoth. Dean then tossed the seal to Ophelia who uses the seal to reverse the spell that Glythur prevented from entering.
He watches as Glythur uses his tentacle to drag Yokoth back to their world with the latter screaming in terror as the rift closes behind them.
The Winchesters explained to the Avila siblings they needed the seal to open a rift and they knew the repercussions but were prepared as they were given the item.