You get 200 XP each time you jump in and battle the beast, even if you die! Of course, being dead kind of makes it a moot point, doesn't it? I did it six times in a row just to see what would happen, and how much you have to feed the beast. One feeding it's pretty chancey as to whether you survive, great armor or not. Odds are in the beasts favor. Two feedings, good focus and armor and the odds are closer to your favor. Three or more feedings and you are much more likely to have sweet, tentacle love even if your focus and armor are not in great shape.
The beast isn't a mean monster. If you just listen at night you'll hear the beast "mourning" and whimpering. Rather like a dog who has been neglected, (or feels he/she is neglected), or has been kenneled for some reason at home. They carry on as if you have beaten them and deserted them for life. The beast is a trapped animal who is in the dark, scared, lonely, hungry and in need of food and attention. If you feed it a lot and then go visit it and give it loves, it gives you loves back along with your 200 xp.