The demon Marbas appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, The Book of Spirits, The Book of the Office of Spirits, The Book of Oberon, The Grand Grimoire, Pseudomonarchia Daemonum, and The Lesser Key of Solomon
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Fifth Spirit is Marbas. He is a Great President, and appeareth at first in the form of a Great Lion, but afterwards, at the request of the Master, he putteth on Human Shape. He answereth truly of things Hidden or Secret. He causeth Diseases and cureth them. Again, he giveth great Wisdom and Knowledge in Mechanical Arts; and can change men into other shapes. He governeth 36 Legions of Spirits. And his Seal is this, which is to be worn as aforesaid.
The demon Varbas possibility the same as this demon.
Timeline of related events
1536Publication of De praestigiis daemonum (lists this demon)
1563First known appearance of The Book of the Office of Spirits (lists this demon)
1577Book of Oberon completed (lists this demon)
1584Publication of The Discoverie of Witchcraft (lists this demon)
1641Publication of the Lesser Key of Solomon (lists this demon)