Volo's Excruciating Fart

Volo created this spell by accident, while trying to figure out the recipe for the hottest chili on the planet. He documented it, and published it in a cookbook that ended up in the hands of an unknown wizard with a twisted sense of humor. The wizard turned the recipe into an offensive (and we mean offensive) spell, and gave Volo the credit.

The caster flicks the material component at the targeted creature, and the creature must make a Constitution saving throw. If on the first round the target makes their saving throw they immediately avoid the effects of the spell. If the target fails their saving throw the double over in gastric agony. They suffer 1d4 necrotic damage and their movement is halved. They also have disadvantage on any attack rolls or skill checks. The target will continue to take 1d4 necrotic damage, and suffer all the afore mentioned effects each subsequent round.

At the begging of the effected target's turn they must make a constitution saving throw, this time at a DC of 10. If they fail their saving throw the effect continues until the target either makes their saving throw or their hit points drop to 0.  If the effected target either makes their saving throw or their hit points drop to 0, they violently pass gas, creating a 3rd level Stinking Cloud spell centered on the targeted creature. If the targeted creature survives, they do not suffer any more of the effects of the initial spell but must now deal with the after effects of the stinking cloud.

Despite popular opinion, lighting a match doesn't help.

Any creature that effectively does not have a digestive system is immune to this spell's effects. This includes Elementals, Oozes, Constructs and certain types of Undead.

Incidentally, Volo has gone on record as taking personal offense that this spell has been accredited to his name.