A replacement effect is an effect (usually an interrupt) that replaces the resolution of a triggering condition with a different means of resolving that same triggering condition, but in such a manner that the triggering condition is still considered to occur. The word "instead" is frequently indicative of a such an effect. After all interrupts to the triggering condition have resolved and it is time to resolve the triggering condition itself, the replacement effect resolves instead. If multiple replacement effects are initiated against the same triggering condition, the most recent replacement effect is the one that is used for the resolution of the triggering condition. (See Replacement Effects.)
You can trigger multiple copies of Crannogmen in the same challenge if you wish to return them to shadows. However, only one character will be killed (the one chosen last).