Saruman's ring

[Zigûr]

Saruman was (like Sauron in fact) a Maia of Aulë the Smith, whose power lay in crafts and arts and things made with hands and with skill, and in the substances of which Arda was made and the shaping thereof. Saruman's own power seems to have been similar - his attempts to forge a Ring and his knowledge of Ring-lore for instance, and in the perversion of these gifts his breeding of hybrid Orc-Men and the industry of Isengard. Saruman's ring is an interesting matter because of how briefly it's mentioned and then never referred to again. I always assumed that this, combined with his desire to find the One, implied that his own ring was only an experiment which had not particular succeeded. It's worth remembering that there were elements of Ring-lore which were lost after the Second Age. Consider Professor Tolkien's remarks about Saruman in the Foreword to the Second Edition of The Lord of the Rings when he speculates on a deliberately allegorical plot focused on the Second World War: "Saruman, failing to get possession of the Ring, would in the confusion and treacheries of the time have found in Mordor the missing links in his own researches into Ring-lore, and before long he would have made a Great Ring of his own with which to challenge the self-styled Ruler of Middle-earth." (xv)
While the shadow and will which emerges from Isengard during the events of Book Three is ambiguous as to its source, I would suggest that it is not related to the power of a ring so much as it is within the power of an Ainu (even, apparently, as a Wizard) regardless: remember that during the War of the Ring Sauron was also able to invigorate his own forces and oppose the wills of his enemies even in the absence of the One Ring.