Hi,
I believe items should have a new line added to their infobox, that displays whether that item has a relic counterpart and if so, links to it.
Example:
{{item infobox
| type = Hat
| tooltip = Boom?
| rare = 0
| cost = {{cash|125,000}}
| id = barel
| canrare = Yes
| drop = 1:280
| cancraft = Yes
| loottable = 4
| relic = [[DANGER ZONE|Yes]]
}}