Spoilers follow- I think it's just supposed to represent the end of Bhaal. All the Bhaalspawn were supposed to be dead to revive the god and now there's some destruction, carnage and the end of that for the old, dead god of murder.
Hm, I figured it had something to do with you finally reaching your end and becoming a God. IIRC the mountain have stone figures all the way to the top, and we all know from the BG ending movie that those represents fellow bhaalspawns...
I figured it was to symbolize your victory over your siblings and achieving something they could not - and of cource becoming a God must have some fancy beam-like effect. ^^;