At the beginning of combat, Duke Ulder Ravengard triggers, giving Éomer an additional instance of myriad until end of turn.
Declare Éomer as an attacker.
Each instance of Éomer's myriad ability triggers, creating at least two token copies of it tapped and attacking each other opponent.
Put all Éomer tokens into your graveyard due to the legend rule.
When the Éomer token(s) die, each Éomer triggers, untapping all creatures you control and causing you to get at least two additional combat phases after this one.
Repeat from step 2 each combat phase.
Once you have an arbitrarily high number of combat phases, you may have Duke Ulder Ravengard give myriad to any other creatures you control.

Near-infinite ETB

Near-infinite LTB

Near-infinite death triggers

Near-infinite combat phases

Near-infinite untap of creatures you control

Near-infinite mana creatures you control can produce

Duke Ulder Ravengard

Éomer, Marshal of Rohan

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.

Near-infinite combat damage

Port Razer

At the beginning of your combat phase, Duke triggers, giving Blightsteel Colossus haste and myriad until end of turn.
Declare Blightsteel Colossus as an attacker.
Blightsteel Colossus' myriad ability triggers, creating a token copy of itself attacking each other opponent.
Deal combat damage with each Blightsteel Colossus, giving each opponent eleven poison counters.
Each opponent loses the game due to having ten or more poison counters.

Each opponent loses the game

Blightsteel Colossus

At the beginning of your combat phase, Duke Ulder Ravengard triggers, giving Mondrak haste and myriad until end of turn.
Declare Mondrak as an attacker.
Mondrak's myriad ability triggers, creating two token copies of Mondrak that are tapped and attacking each opponent.
Put all Mondrak tokens into your graveyard due to the legend rule.
When the Mondrak tokens die, Ratadrabik of Urborg triggers that many times.
Resolve the first trigger, creating a number of nonlegendary token copies of Mondrak equal to 2^(the number of Mondraks you control) that are 2/2 Zombies.
Repeat step 6 for each remaining trigger.

Near-infinite creature tokens

Ratadrabik of Urborg

Mondrak, Glory Dominus

At the beginning of your combat phase, Duke Ulder triggers, giving a Bloodthirster haste and myriad until end of turn.
Declare the Bloodthirster targeted in step 1 as an attacker.
The Bloodthirster's myriad ability triggers, creating a token copy of it tapped and attacking each other opponent.
Deal combat damage to an opponent using a Bloodthirster token, triggering the token, untapping it and causing you to get an additional combat phase after this one.
Move to your next combat phase. You will not exile the Bloodthirster tokens due to The Master.
Repeat.

Infinite ETB

Infinite combat phases

Infinite creature tokens with haste

Infinite combat damage

The Master, Multiplied

Bloodthirster

At the beginning of your combat phase, Duke Ulder Ravengard triggers, giving Adrix and Nev haste and myriad until end of turn.
Declare Adrix and Nev as an attacker.
Adrix and Nev's myriad ability triggers, creating two token copies of Adrix and Nev that are tapped and attacking each opponent.
Put all Adrix and Nev tokens into your graveyard due to the legend rule.
When the Adrix and Nev tokens die, Ratadrabik of Urborg triggers that many times.
Resolve the first trigger, creating a number of nonlegendary token copies of Adrix and Nev equal to 2^(the number of Adrix and Nevs you control) that are 2/2 Zombies.
Repeat step 6 for each remaining trigger.

Combos with "Duke Ulder Ravengard"