Being referenced in the Elder Pharos sonnet of Fungi From Yuggoth (note that in the sonnet, last is in lowercase, so he is the last Elder One, rather than it being a specific title). It is briefly described as wearing a yellow-silk mask that bulges over unearthly features and that none know what it really is, although it comports itself as if human. It has also been refered to as the Tcho-Tcho Lama of Leng, although it has also been identified as existing in the Dreamlands amongst the semi-human Men of Leng. All that is known for certain about it is that it dwells in the Elder Pharos.
If it is to be located in the Dreamlands, then it is likely that it is one of the Moon-beasts that rule the Men of Leng. However, if it is to be found in the waking world, its identity is unclear, although Elder One could imply a link to the Elder Things. It has also been identified as Nyarlathotep or a human hosting the psychic avatar of Nyarlathotep.
Its appearance would imply an idenfitication with The King In Yellow, but there is dispute over when HP Lovecraft actually encountered the Yellow Mythos, potentially ruling out any deliberate connection. One alternate name for this being - The High Priest Not To Be Described - is reminiscent of Hastur's sometimes sobriquet of The Unspeakable.
The last Elder One also appears in Lovecraft's Dreamlands stories Celephais (in which it is termed The High Priest Not To Be Described and said to live alone in an ancient monastery) and The Dreamquest of Unknown Kadath (which repeats what Celephais says of it, strongly hints that it is a Moon-beast and states that it prays to Nyarlathotep).