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When Silverclad Ferocidons's triggered ability resolves, first the player whose turn it is (if that player is an opponent) chooses which permanent they will sacrifice, then each other opponent in turn order does the same, then all chosen permanents are sacrificed at the same time. Players will know choices made by earlier players when making their choices.
If creatures an opponent controls are dealt lethal damage at the same time that Silverclad Ferocidons is dealt damage, those creatures will be destroyed before that player chooses a permanent to sacrifice.
If multiple sources deal damage to a creature with an enrage ability at the same time, most likely because multiple creatures blocked that creature, the enrage ability triggers only once.
If lethal damage is dealt to a creature with an enrage ability, that ability triggers. The creature with that enrage ability leaves the battlefield before that ability resolves, so it won't be affected by the resolving ability.
