From Portals of Phereon Wiki

Giants[ | ]

Giants are a special optional combat challenge, most of them within random portals. They only attack you if you step on the same tile as them. Giants are stationary and can sometimes be "hidden" by roaming enemies. All wild Giants will be enormous size, thus they will have high health and hit like a truck. It is recommended to be fully prepared before attempting to fight them, as a single boss can wipe your entire team.

Killing and looting their corpses is how you obtain some Special Items.

Each new day, when you enter a portal, it rolls for a boss encounter, so if you're hunting for them enter each portal every day.

Killing a Giant boosts the entire party's max morale by 1, you can even get more max morale from the kraken and lava fish if you don't harvest them.

The Horror does not follow regular Giant rules, and instead only shows up in a single story fight in the alternate ending. The Horror is not recruitable.