He will take on human shape if commanded to do so, but this will cause musical instruments to be heard but not seen. Trees sway at the sound of his voice, and he gives people the power to make trees fall and also gives excellent Familiars.
From Demonologia ( August 2003 )
Amdusias (also spelled Amduscias and Amdukias) is one of the Demons of hell described in the Goetia, one of the books of the manuscript grimoire known as the Lemegeton or Lesser Key of Solomon. He is mentioned by Johann Wier in his Pseudomonarchia daemonum , and by Reginald Scot in The Discoverie of Witchcraft
In Collin de Plancy's illustration, Amdusias is crowned to show that he is a duke of hell. He has been given a horse's head to go with his single horn, because unicorns are usually represented in the shape of a horse. Two trumpets lie on the ground, and a third trumpet is slung over his shoulders.
From “The Goetia: The Lesser Key of Solomon the King” (1904) Written by S.L. MacGregor Mathers
AMDUSIAS, or AMDUKIAS. – The Sixty-seventh Spirit is Amdusias, or Amdukias.
He is a Duke Great and Strong, appearing at first like a Unicorn, but at the request of the Exorcist he standeth before him in Human Shape, causing Trumpets, and all manner of Musical Instruments to be heard, but not soon or immediately. Also he can cause Trees to bend and incline according to the Exorcist’s Will. He giveth Excellent Familiars. He governeth 29 Legions of Spirits. And his Seal is this, etc.
From the “Pseudomonarchia Daemonum” ( 1583 ) Written by Johann Weyer (Johann Wier)
Amduscias a great and a strong duke, he commeth foorth as an unicorne, when he standeth before his maister in humane shape, being commanded, he easilie bringeth to passe, that trumpets and all musicall instruments may be heard and not seene, and also that trees shall bend and incline, according to the conjurors will, he is excellent among familiars, and hath nine and twentie legions.
Original Text : Amduscias Dux magnus and fortis: procedit ut unicornu: in humana similiter forma, quando coram magistro suo se sistit: Et si præcipiatur, efficit facile ut tubæ and symphoniæ omniaque musicorum instrumentorum genera audiantur, nec tamen conspectui appareant: ut item arbores ad exorcistæ genu se inclinent. Optimus est una cum famulis. Imperium habet vigintinovem legionum.
From the “Dictionnaire Infernal” (edition of 1863 ) Written by Jacques Auguste Simon Collin de Plancy
Amduscias governs twenty-nine legions. His true form is as a unicorn, but appears as human form when summoned. He will give concerts if commanded, and is accompanied by the sound of trumpets and other musical instruments. Trees also bend to his command.
Original Text : Amduscias, grand-duc aux enfers. Il a la forme d'une licorne; mais lorsqu'il est évoqué, il se montre sous une figure humaine. Il donne des concerts, si on les lui commande; on entend alors, sans rien voir, le son des trompettes et des autres instruments de musique. Les arbres s'inclinent à sa voix. Il commande vingt-neuf légions.