Your shrine loses 25 hp and all champions become Pacified 3 (This champion cannot attack or use abilities to attack, and cannot engage other champions. This cannot be cleansed). Any champions that enter play before the effect ends also become Pacified for the remaining duration.
PoxNora Release Set
This was used in several trick decks, most using it in a "truce lock" manner, but these were rather rarely encountered. Note champions who are pacified can't use attacks or attack abilities, meaning their ability to do damage to anything is usually stopped by pacified. (This may be why the "your shrine takes 25 damage" part was added on after a few years.)
Note truce ends after your turn, meaning your opponent has the first chance to attack your champions. This is a big drawback (and one not present in newer spells like Reclaim) so Truce is rarely run outside of trick decks.