A Helm of Dragons is typically an open faced helm, of a type popular in the
late 12th and early 13th centuries DR (roughly 1180-1220).  They were
created mostly by the sect of the Cult of the Dragon that was active in the
Thunder Peaks at that time (with the activity centered around the Temple of
the Dragon Oracle hidden in the Thunder Peaks.  They are characterized by a
small set of wings (which extend from the ear to the back of the helm and
never reach above the helm) and a small draconic head of indeterminate type
upon the helm's forehead piece.  The secrets of their creation were lost
after their creators were killed in the battle against Lathander in the
latter half of the 13th century.

Powers
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The helms were created for the use of Cult scouts.  As such, they had two
powers:

1)  A Helm wearer's senses were sharpened to the level of a mature dragon.
As such, they could effectively detect invisibility within 120 yards,
through a combination of heightened senses of smell, hearing, and sight
(coincidentally, rogues attempting to hear noise while wearing a helm of
this type suffers no penalty to his attempts to do so).

2)  The second power, largely unknown to those outside the Cult cell that
created and utilized these Helms, was to empower those in some type of
Scale mail (be it metallic, regular leather done in a scale like pattern
like the armor sometimes worn by the faithful of Silvanus, or those wearing
dragon scale armor) to be Protected from Normal Missiles (akin to the
draconic immunity and/or the 3rd level wizard spells).  Again, as most
warriors wear better armors than scale, this power is largely unknown.