The Spirit of Gargos is one of the 10 buffs that Gargos possess and gives to an Omen.
Shadow Lords File[]
"The Spirit of Gargos significantly extends the duration of his Instinct Mode, allowing him to hold Stoneskin for much longer. When Gargos activates Stoneskin, it is always advisable to use strong attacks to pierce his armor!"
In-game effect[]
As for Omen, his Instinct will be simply prolonged, while as for Gargos, his Stoneskin lasts longer than usual, which sometimes, can make him "explodes" Stoneskin and rush to the player, damaging him/her, if not finishing the fight.