Historical Notes
The Battle of the Trident was fought at what would come to be called the ruby ford.
The rebel host was smaller than the royal host, which included contingents from the Reach and Dorne and was said to have numbered 40,000 strong with a tenth of them being knights and the rest archers, freeriders, and foot, but the rebels were more "battle-tested". There Robert and Rhaegar fought one another in single combat while battle raged around them.
When Robert drove his warhammer through Rhaegar's ruby-encrusted breastplate, members of both armies rushed to collect fallen rubies from the river while the rest of Rhaegar's forces routed.
Pennant
Rhaegar Targaryen
Prince
Command level 5 
War Council level 0
Standard 
Robert Baratheon
the Usurper
Command level 6
War Council level 0
Victory Conditions
9 Banners
The side which gains 9 flags first or kills the opposing commander wins the battle.
Briefing
Prince Rhaegar and Robert Baratheon finally meet each other with their hosts. They are actively seeking each other out to find out which one is mightier, Robert's war maul or Rhaegar's blade of valyrian steel.
Special Rules
The river is fordable from the fork where all branches join.
Banner bearers of the units including Rhaegar and Robert can be killed only by each other.
Killing hits from other units are considered black flags instead. Boths units are also considered bold all the time. (I.E. Rhaegar can only be killed by Robert and vice versa)