Official rule text
73.2 When a player is instructed to gain a relic, they draw the top card of the relic deck and place it faceup in their play area. a If there are no cards in the relic deck, they do not gain a relic.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 73.2