This page or section includes possible gameplay spoilers or secrets. Tread carefully.
Between the Temple of the Art and Magical Temple is a strange triangular shaped structure, that repels projectiles. This is the Gate Guardian. It is generally inert and unresponsive to almost everything, but it can be awoken with the right tools. The artwork on its surface depicts several hints as to what is required to awaken it.
This content includes possible spoilers or secrets. Tread carefully.
Throwing 3 of any Egg at the triangular Gate (or casting 3 Summon Egg or Summon Hollow Egg) at it begins the boss fight. The structure splits into 4 pieces, and the gently projectile repelling field is transformed into a damaging aura on each construct.
Once awoken, the Gate Guardian splits into four very similar constructs (named Veska, Molari, Mokke, and Seula), which fly around haphazardly trying to get close enough to the player to cause damage. Each possesses an ominous red aura which, unlike in their inert state, will deal rapid curse damage to any living things that come within its range.
Their auras are also capable of removing terrain quite quickly, although they have no ranged abilities. The fight typically consists of evading their advances whilst trying to deal enough damage to bring them down. Each of the four constructs is an obstacle to the others, so in most cases only one or two will manage to be a threat at any given time, unless the player is in sufficiently open space to allow them all to approach at once.
The terrain-clearing effect they emit on waking up creates a sort of arena that is somewhat helpful to the player's mobility, but also assists the guardians in being able to target the player all at once. In more cramped quarters, they struggle to reach the player all at once. Despite their ability to erase terrain, they will tend to prefer going through already accessible routes if any are available, and they tend to avoid moving too close to each other.
Because they are physical entities, similar to Verikristalli and the other floating crystal enemies, their physical form can be damaged directly with matter-destroying spells. If enough of their physical form is destroyed, they will die, although killing them in this manner does not award the kill to the player (the kill will not be recorded in the Progress menu).
For a more direct kill, explosion damage typically is the most effective at penetrating their otherwise quite sturdy defenses. Slice damage is also a very effective method, although there may be some issues with the disc projectiles interacting oddly with the guardians' rigid-body forms, as most slicing projectiles can bounce off of solid objects. Chainsaw can be quite an effective option with a setup that allows it to be dealt at range, but close-range attacks should be avoided.