Nimbus of Pathos states:
When this effect ends, the creature immediately drops to 0 hit points and is dying. Once you use this feature, you can’t use it again until you finish a long rest.
By creature, RAW, obviously means Druid but is it intended to effect the druids original form HP pool?
Your game statistics are replaced by the statistics of the beast… when you transform you assume the beasts hit points and hit dice, When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form.
From what I can find dropping to 0 HP and HP becoming 0 are treated the same, so the bard ability should affect the beast form and only the beast form considering it’s HP is not temporary but a replacement stat.