El Oscuro

"The dark ancient force worshiped by the Oscuridos. With many magic attacks and awful surprises, you'll be dead twice before you notice your body is vaporized."

El Oscuro is the final of 4 bosses found in Rise of the Triad. He is fought at the end of the fourth episode, The Slow and the Dead, and has multiple forms.

Normal Form
Aside from the additional dark brown hooded robe (the inside of it coated with red) and black gloves, El Oscuro wears pretty much the same robed clothing as the rest of his monk followers. He has several different forms of attack, most of them only used to counterattack the player when hit with their arsenal of weapons (save for the Pistol and MP40). That being said, it should be noted that El Oscuro is invulnerable to all of the players weapons. He also has the ability to fly, although it seems to be more of a hover.

Strategy
You will encounter El Oscuro's normal form at the end of his lair. From afar, you'll be able to distinguish him from the other monks by his ability to float. While you may be confused at first, the key to defeating him is simple - Do not attack him. Firing at him is useless, and can often hurt you when he retaliates. Instead, keep your distance and avoid his attacks, making sure to get rid of other enemy distractions as well. Eventually, El Oscuro will expend all his power and transform into his Snake Form, fleeing the level. You will need to follow him through a winding canyon, and confront him one last time in his dark nest.

Strategy
You'll encounter El Oscuro again in his Snake Form in the room with the four large pillars (E4A10). He will chase after you as soon as he spots you, so you'll have to keep your distance as much as possible. The key to defeating this form is different than of his Normal Form. To kill him, you'll have to make sure your weapons hit him while he is over the cluster of Fire Pits located near the center of the area - his color will turn red while he is in this vulnerable state. He will not take damage if he is anywhere else in the area. After inflicting enough damage onto him this way, El Oscuro will eventually die in a large explosion.

However, unless you want to repeat having to destroy him again, you'll make sure to destroy all of his larvae.

Oscuro Larvae
The Oscuro Larvae are small eggs with a spitting image of El Oscuro inside them, which are scattered throughout the final level (E4A10).

These larvae are unhatched descendants of El Oscuro, and their fate determine yours. In other words, if you let even one survive before destroying El Oscuro, you will receive a bad ending. The story will inform you that because you failed to kill all of El Oscuro's larvae, one of them went and destroyed the world 30 years later. You will then have to re-enter the final level, destroy all of the larvae, and destroy El Oscuro once again.