At the beginning of your combat phase, Duke triggers, giving
Port Razer haste and myriad until end of turn.
Declare
Port Razer as an attacker.
Port Razer's myriad ability triggers, creating a token copy of
Port Razer attacking each other opponent.
Each
Port Razer deals combat damage, triggering themselves, untapping each creature you control and causing you to get an additional combat phase after this one for each trigger.
At end of combat, the token
Port Razers' myriad abilities trigger, exiling them.
Repeat once for each opponent.
Results
Near-infinite combat damage.
Near-infinite combat phases.
Near-infinite ETB.
Near-infinite LTB.
Near-infinite untap of creatures you control.
Near-infinite mana creatures you control can produce.