The final boss fight. Let's detail the whole thing, shall we?
You start in the middle of a square arena where waves of monsters, starting from the weak yet annoying Kleer all the way to waves combining both categories of Biomechanoid with Werebulls, Beheaded Kamikaze and Kleer, all for you to mow down as the ammo pack and health items keep you alive. Then the door opens and you're faced with a big corridor.
Then, just as you exit the area, in comes Ugh-Zan III, a towering, bipedal, multiarmed monster chasing you from behind. At this point, your mission is to reach the pyramid while avoiding him (as he has a Healing Factor and your weapons can't deal enough damage to counter this perk of him)... And several waves which not only include the aforementioned enemies but also the other types of beheaded soldiers and both types of Arachnoid.
Once you reach the pyramid, Sam places the relics in position and is brought to the upper level of the pyramid, where UZ3 spawns. Now it's all between you and UZ3, no more enemy waves. Your objective is to get him aligned with the only thing that can keep him down for the count: a super laser from his spaceship. It takes quite a bit to make it work (and unfortunately he still keeps his regeneration ability) but once the laser takes him down for good, Sam boards the ship and gets ready to continue... until the beginning of Serious Sam: The Second Encounter, that is.
