Add a subrule to Einherjar named Special Attributes. It shall contain the following text

An Einherjar may have up to three special attributes. A special attribute is some text that augments the statistics of any specifically named Einherjar, or any Einherjar with a specific statistic value, if certain conditions are met. Unless otherwise allowed by the Ruleset, a special attribute can only be added when an Einherjar is first made a candidate. The special attributes shall be listed in the proper Einherjar’s section on the Einherjar wiki page.

It looks like this will be covered already in the Einherjar Candidates rule where it says “suggest the relevant attributes and statistics of that historical or fictional character”.