Baernaloth
The legendary baernaloths (I pronounce the first syllable either "bear" or "Burr"), sometimes spelled baernoloths, are among the chiefest of all mysteries in the multiverse.
They claim to be the eldest fiends. They claim to have created all of the other fiends, including the yugoloths, demodands, baatezu, tanar'ri, hordlings, nagals, and night hags. Some even claim they spawned the ancient Baatorians and obyriths.
Nobody knows if this is true, of course, but regardless, the baernaloths seem to be some of the oldest beings in the multiverse. The depths of their knowledge are unfathomable, and many fiends legitimately provide them deference. It is not uncommon for even very powerful fiends to seek them out for knowledge and wisdom.
Alignment
The baernaloths are Neutral Evil, though the urgaloth Anthraxus allegedly has a retinue of baernaloth viziers who are Chaotic Evil.
Physical description
Baernaloths are the stuff of nightmares. They are 8 feet tall, gaunt, and lanky, with freakishly long limbs and grey skin. They look emaciated and skeletal. Their heads lack skin and are instead bare skulls resembling that of a ram or bovine.
Baernaloths might not look particularly impressive, but believe me when I say you don't want to fight them.
Known baernaloths
No one has any idea how many baernaloths are still alive. Estimates range from zero, to one, to a handful, to well over a dozen.
One of the only entities we think might be a baernaloth is Apomps.