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 5 for each remaining trigger.
This explosive combat loop relies on exponential token multiplication to quickly swamp the board. When you declare Mondrak, Glory Dominus as an attacker, Legion Loyalty grants it myriad to flood the combat step with attacking copies. Thanks to Mondrak, Glory Dominus's own replacement effect, the number of tokens entering the battlefield scales exponentially rather than just doubling. The legend rule promptly sends these extra copies to the graveyard, triggering Ratadrabik of Urborg to revive them as nonlegendary 2/2 Zombie creatures—which, crucially, bypass the legend rule and survive to repeat the process with even greater force.