The plates that sheathe these boots mimic the shapes of leaves. When equipped in a set with ancient elven armor, helm, and gloves, the character gains a bonus to defense.
Found by asking Ser Bryant in the Lothering chantry if there is other assistance he can provide. He will give the the Warden a key to a locked cabinet in the chantry that contains the boots.
In the PC version, the boots never appear in the cabinet, rendering them unobtainable by normal means. See Ancient Elven Boots#Bugs for workarounds.