Solemn Trinity
“The gigants are made up of three distinctive classes: the ruling class of gigas, the slave class of giants, and the hecatoncheires─a second class of slaves who have spent the past several centuries trapped in a sealed mine. The enmity between the classes runs deep, and when any two converge, sparks will fly. But, when all three converge...the results can be devastating.
Parties from all three of the gigant classes have stumbled onto a dig site of the Sons of Saint Coinach. The researchers have all fled to safety, but the artifacts they discovered remain on the field. Slay the intruders while protecting the relics from needless destruction.— In-game description
Solemn Trinity is a level 40 guildhest introduced in patch 2.0. This duty provides practice with working with a full party, multiple tanks, and multiple healers. Note that because this is the only guildhest that requires an 8-player party, queue times for this can be fairly lengthy.
Rewards: 
5,640 experience, 
240 gil
Strategy
Solemn Trinity pits the players against waves of incoming giants.
Players must not only survive the giant attack but also prevent the giants from destroying the crates in the area. The giants will enter the arena from 3 sides, north, east and west. Tanks should spread out and take the aggro of incoming giants.
Healers should stay in the center of the room so they can heal all the players around. DPS should run around and kill the giants wherever they come.