So I was getting that nostalgia game started around 1985 and my buddy says something like "Three brown books, huh? You know that rule on Other Character Types in Men and Magic where it says you can be a Balrog, just starting as a small one? I want to be a Balrog!" and a bit later to answer how he can be around other characters, "He was raised from an egg by a blind druid, so he's somewhat socialized, in a Deadhead sort of way." And I said something like "Ookkaaayyy, I'll work on that," and went off and drew up a playable Balrog class.
It had to grow, it had to at least start out as something that other characters could play alongside, and it had to feel different than a fighter with wings and a big whip, immolation and magic resistance. And as it levels up, the Balrog nature has to become stronger, and make the character less able to be part of an adventuring party.
I just found that sheet of notebook paper while digging through old D&D notes, so here it is.
Approximate rate examples
Eat a sword +1: 100P
Eat a sword +2: 400P
Eat a cursed potion: 50P, suffer from gas for 2D6 turns, starting after 1D6 turns.
Eat a magical monster or magic user- HD squared x 10P
Absorb a spell - level of spell x level of caster P.
Use of the whip and immolation -- When not striking great blows with the sword, may attack as 2 weapon user for one attack per round, striking once with each weapon. Sword and whip is the habitual form so no penalty for this two weapon combination. Whip does 1D3 and target must make a DEX save versus entrapment for the next round + immolation damage if immolation roll is made. Can alternatively make multiple whip attacks, one per action, instead of using the sword, but counts only whip 1D3 and immolation damage, and only entraps the last one. IMM is the number or higher to roll on 2D6 to immolate each round.
Magic Resistance: BMR is Base Magic Resistance. This is the chance to resist and absorb a spell cast by a first level magic user. Each higher level of caster reduces this chance by 5%. So a 10th level MU casting a web on a 4th level Balrog has 20% chance of the spell being absorbed.
Berserk: This is the percentage change of battle madness setting in for 2D6 rounds when surprised while entering combat, if foes outnumber friends by hit dice estimate at the start of a battle, when hit for 10% or more of HP, etc. While berserk, the Balrog will attack at +2 and immolate at +2, and ignores all wound effects until the berserk state wears off, and only then rolls checks against CON. A berserk Balrog will attack friends if all foes are defeated, until the effect wears off or it is magically calmed.