[RPG] How much damage can a druid Wild Shaped into a spider take without dying instantly

When you Wild Shape/Polymorph you "assume the beast’s hit points" thus essentially creating a new, separate pool of HP from your own, original form similar to how Temporary Hit Points work, as Alexis Wilke has stated.

Damage taken in animal form doesn't affect your original form's HP unless you're dropped to 0 HP in animal form and there's excess damage. Nowhere is it suggested that max-HP reduce would work any differently. Because Wild Shape/Polymorph gives you a new pool of HP (as supported by Jeremy Crawford in the link below), only that pool is affected by the reduction.

So, using your example, if a PC has 30 HP in their original form and transforms into a beast that has 50 HP, the PC effectively has 50 HP. If the PC has their max HP reduced by 30 while transformed then they don't die as a result of having 0 HP because they're using the beast's HP and they still have 20 HP left in that pool.

As for whether the max HP reduction carries over to your original form when you revert, according to Jeremy Crawford, lead rules designer, the answer is no:

Jonathan Longstaff
^@pukunui81
@JeremyECrawford What happens when a wildshaped druid that has had its HP max reduced reverts back to normal? Does the reduction carry over?

Jeremy Crawford
^{@JeremyECrawford}
Wild Shape—a reduction to hp maximum doesn't carry over from your beast form to your true form or vice versa.

Unfortunately, there are no rules for this circumstance, so if our itsy-bitsy-spider-druid runs out of Wild Shape while trying to climb up a downspout, we have to come up with a ruling, and there are three routes here:

1. The generous route is to let the druid stay shaped until they exit the downspout (or other space that can't fit their humanoid form at all), as the unshaping fails completely. If they are 0 HP while shaped in this case, then they stay at 0 HP, shaped, but unconscious, and have to make death saves just like any other non-stabilized unconscious PC.

2. The stringent route is to simply eject the druid from the too-small space; this smacks of video-game physics, but skews the balance away from the druid in that he can't be an itsy-bitsy-spider-druid hiding in a downspout for months or years. If ejecting him from the space is impossible (say, if someone sealed him into a small box while in itsy-bitsy-spider-form and FedEx'ed him), then you either choose a different route, or rule that ejecting the druid either passes through or destroys any obstacles that prevent ejection from the space.

3. The possibly-painful route is that the druid tries to unshape, and starts to enter human form in a space too small to contain a human, so he takes crush/constriction damage (and so does the object) until either the druid dies a horrible death (the continuous nature of this damage would make it impossible for him to make his death saves), or the object that was trying to contain the unshaping druid is destroyed, leaving a fuming druid amid a pile of shredded cardboard, a ripped downspout, or what-have-you. This would be analogous to the drowning rules, as it'd be incumbent on the player to avoid putting their character into such perilous situations, and considered only a fair consequence that the druid might die for screwing up that badly.

Of course, if the druid has another use of Wild Shape at hand, they can avoid all this hassle by burning that use of Wild Shape to avoid unshaping at all for the time being.