Red Devil

It joins the Underworld population the moment any mechanical boss is dead, replacing the demons you were previously walking past. That is one kill away from a hell layer you could cross safely, so the change usually catches people mid trip rather than at a point where they prepared for it. On the oldest console version the check is looser and one can appear before Hardmode if a mechanical boss somehow died first.

It flies at you the way a demon does and throws a burst of purple tridents. Touching the devil itself barely registers. The tridents punch through endgame armor. They come out in the same rhythm demons use for scythes, but slower and at a fixed speed, which sounds like an improvement and is not. Slow projectiles hanging in the air over a lava lake are harder to route around than fast ones, because they are still there when you double back.

Because contact damage is so much lower than projectile damage, deliberately bumping into one is often correct. The invulnerability window from that touch carries you through the volley for a fraction of what the volley would have cost. It only works if you can absorb the contact comfortably, which by this point in a run most builds can.

They do not resist knockback much, so a Megashark or Chain Gun can pin one in place. It keeps firing while stunned, so this buys you a stationary target rather than safety. In Master Mode the tridents can kill outright in one or two hits regardless of defense, and at that point the Underworld becomes somewhere you cross at speed rather than somewhere you fight.

In Remix and Zenith worlds they hand over the Flower of Fire rather than the Unholy Trident. That is the seed's weapon reshuffle rather than a nerf, and in those worlds the flower is the stronger of the two by a wide margin.