Roaring basilisks trait activates even if it has taken damage.
According to the description “If this creature does not take damage from attacks or spells between each of its turns, it afflicts all enemies with Stun for 1 turn.” it should not.
Also this trait is kinda a perma lock if you manage to activate it as stunned creatures do not deal damage unless you find a stun immune.
another issue about those is that if you have 2 roaring basilisks and they start om top of battle queue then the first one will not cast its mass stun but the second one will cast it already. This is kinda odd and im not too sure if it really a bug or some sort of feature.
Version 2.0.0 on Windows 10