James Jacobs
Adventurer
I suspect that the shortage of disease-flavored demon lords is mostly due to the fact that the daemons picked up that schtick very early on, in the 1st edition MM2. There wasn't a lot of demon lord development happening at all during 2nd edition, and a lot of 3rd edition's demon lord development was focused on recapturing the feel of the 1st edition demon lords (and spending some necessary evil time rebuilding and refocusing on extant lords).
Anyway, were I to choose, I'd pick one of the following as the disease lord:
Bayemon: Demon lord of the afflicted.
Bechard: Demon lord of decomposition
Zivorgian: Demon lady of carrion and vultures
All of these are pretty good choices, since there's not been much done with them yet and their established interests are disease friendly.
HOWEVER: My best choice would be Yibyiru, the Rancid Lady of Bitter Bile. She's technically a demon lord associated with poison, but we've already got enough of those; were I still doing Demonomicon stuff, I'd probably recast her as the demon lord of disease, poison, and atrophy.
Anyway, were I to choose, I'd pick one of the following as the disease lord:
Bayemon: Demon lord of the afflicted.
Bechard: Demon lord of decomposition
Zivorgian: Demon lady of carrion and vultures
All of these are pretty good choices, since there's not been much done with them yet and their established interests are disease friendly.
HOWEVER: My best choice would be Yibyiru, the Rancid Lady of Bitter Bile. She's technically a demon lord associated with poison, but we've already got enough of those; were I still doing Demonomicon stuff, I'd probably recast her as the demon lord of disease, poison, and atrophy.