In a battle with any Monster with the feature "Legendary Resistance" (ex. Ancient Blue Dragon), with which the monster can choose to automatically save a failed saving throw, how does this then work when a spell is cast with no saving throw (ex. Power Word Kill)? Does the Legendary Resistance automatically render the spell null and void then? Or does the spell win out because there is no saving throw? Or is it simply a matter of DM discretion and can go either way?
If the creature with Legendary Resistance has less than 100hp, it would kill them.
Power Word Kill has no saving throw, so Legendary Resistance does not apply.
However, a creature with Legendary Resistance very likely has more than 100HP. If it does have less, it would be a phenomenal finishing move on them (at the cost of your 9th level spell slot.)