Need some help with how to rule something my player is currently doing.

Spoiler warning for minor part/ optional encounter from Out of the Abyss module

TLDR: Path of the giant barb in HUGE size leaping onto LARGE size enemy (beholder) floating over 100ft drop. Do they plummet? How does this play out? Are there rules anywhere for this?

So for more detail/ overview of fight that lead to this point. Anyone who reads all this thank you in advance I know I'm bad about writing novels instead of posts.

The party just left Mantol-Darith with a couple new npcs. They brought pebbles along as the ranger made good friends with him harvesting eye stalks. Pebbles did the whole "were dangerously close to a purple worm nest" thing and the party diverted at his instruction so pebbles brought them to Xazax the Eyemonger who rises or if that 20x20 100ft deep hole

Fight ensues Xazax rolls a nat 20 on initiative. Round one, 3 rays go out and the paladin get his with a petrification ray and fails his save. His turn comes up and again fails, so its only half way through round 1 and the paladin is gone already.

Fight is continuing and Xazax is on freaking meth or something because he's rolling (randomly with a d10) death, petrification, telekinesis, and sleep rays almost exclusively. He kills 4 of the zent thugs, puts the shield guardian to sleep, and all around he's just pounding on these guys and the 3 npcs i bring in every round from their army to kinda help give him "other" targets/ cannon fodder to chew through.

Round 3 happens and the ranger gets hit with a petrification ray and fails, same on his next turn. So now that's 2/5 PCs down and out for the count.

So at the very end of round 3 the Duergar, path of the giant barbarian decides to double up and go HUGE size. He then proceeds to jump ONTO and grapple Xazax, who hasn't moved and therfore is still hovering over the 20x20, 100ft drop hole.

It was late and time for fireworks and I admitted im not sure how this would work or if there are rules or what not for it so we agreed to stop there on the almost literal cliff hanger.

Wtf do I do? Is there any actual rules for something like this?

To add, I'm a HUGE fan of "the sacrifice play" (Rogue 1 is my favorite SW because everyone dies in the end, and the end of Looper, and endgame are amazing for this reason) so I have extra rules to allow for that type of decision to be made at anytime when going down. So he very much knows this could very well end in his death and from what he said to other players thats more than fine at this point (he actually seems pretty excited about it honestly) with how bad Xazax is whopping on them. (He's not even bloodied yet but just barely).

So any and all help to try and get this right and/or make it epic is greatly appreciated.

Edit 1: using info back from people the weight of the barbarian is INSANE. at a x2 size and x2 weight for each (2x) enlargement and a weight range of 114-175lbs his weight is.

If x16=1826-2800 If x8x8=7296-11200. (This i believe is the correct RAW formula)

So with that in mind and if the barb successfully rides xazax down and wins the contest to stay on top, should I increase the damage?

I know this is DND5E and not BG3, but with those numbers I kinda feel like the whole "owl bear from the top rope" type attack could/maybe should be taken into account?

Edit 2: nm it was wrong info from another forum. So removed.