Rakdos Firewheeler

When Rakdos Firewheeler enters, it deals 2 damage to target opponent and 2 damage to up to one target creature or planeswalker.

Ever wonder why you never see an old Rakdos cultist?

Ravnica Remastered

When Rakdos Firewheeler enters the battlefield, it deals 2 damage to target opponent and 2 damage to up to one target creature or planeswalker.

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If you can't target any opponent with Rakdos Firewheeler's ability, the ability doesn't go on the stack. You can't have it deal 2 damage to any creature or planeswalker. On the other hand, if you do choose two legal targets but either target is illegal when the ability tries to resolve, the other is still affected as appropriate.