Forged from the three nails used to crucify Christ, this rune multiplies the player’s attack damage by three, but only affects nail weapons.
The trinity is a valuable powerup because it offers a good boost in combat without the insane carnage* and explosives hazard of the quad.
One of the interesting effects is that when the player is carrying the trinity, the NG becomes half-again as powerful as the SNG while using less ammo, but the player has to make the most of it.
Gameplay setups involving a non-secret trinity and limited weaponry can be created to bait the player into ambushes and so forth. Because the player must use nail weapons if they are to benefit from the trinity, it can also be used to encourage the player to deplete their precious nail ammo in predictable ways.
Against most monsters, trinity + SNG = swathe of destruction.
Note that carrying both the trinity and quad has a multiplying effect – the SNG will become devastatingly powerful. This is left to your discretion.
* Not that there’s anything wrong with insane carnage. But we’re talking balanced gameplay here, okay?