A Divine Soul sorcerer in our group is quite fond of using polymorph to change herself into a giant ape (MM, p. 323). At one point in our game last night she managed to grapple a Bheur (Winter Hag) that was riding a Nightmare (flying evil Pegasus). And during the fight, the mount was killed, sending them all into a 1200-ft. nose dive. The fall did 120d6 damage (specifically, 311 damage) to her polymorphed Ape form, which was it turned out exactly 3 points from killing her instantly with the massive damage rule (PHB, p. 197).
An argument about the damage rule ensued, which I felt was squared away with simple math, but it then led directly to another argument over the fact that she got to remain in her Giant Ape form because she still had the spell death ward active on her sheet. This might have just been simple confusion over how much time had passed after their long rest, as in her case she had death ward active for 16 total hours, rather than the normal 8 hours, because of the sorcerer's "Extended Spell" Metamagic option (PHB, p. 102).
The argument then continued, to the entire table's near complete exasperation, as it became a debate over whether or not she should be reverted back to her original sorcerer form because she hit 0 HP (which triggered the death ward), or if she never actually hit 0 HP because death ward keeps you at 1 HP… (Ugh.)
I made a ruling at the table that yes, she's going to continue on as a giant ape at 1 HP despite taking over 300 damage, regardless of how "ridiculous" the other players found it to be, because those are the rules (and also "Because: MAGIC"). But the question of how it works by RAW is now eating at me…
When the death ward spell is triggered by damage, do you first technically hit 0 HP and then go back up to 1 HP? Or do you effectively just never go below 1 HP?
The Sorcerer retains their Giant Ape form with 1 HP
The death ward spell description specifies the following (emphasis mine):
The Sorcerer was the target of the spell. Their hit points were replaced with that of their new form when polymorph was cast; however, polymorph does not remove benefits of spells cast on the previous form.
This means their new hit point pool never drops to 0 hit points.