The Tallassarian Tempest Blade is both a signature card, and a relic, which both forces it into your deck, and limits the card to a 1 off total in the deck.

Does relic mean 1 relic of any specific type in your deck or 1 in total in your deck?

If so, that does limit deckbuilding creativity somewhat (once more relics are released), as it means Sicarius can never have any other relic?

In addition, I've just realized that the core set space marine 'tutorial deck' is illegal (aside from having only 36 cards)

The Iron Halo and the Tempest blade are both relics. Both need to be included in your deck if you take all the SM cards and 1 set of neutral cards.

That could cause some significant confusion.

Relic only matters for in play. Not deck building.

RRG page 10.

Specifically, the rule is "one Relic in play," not "one Relic in your deck."