All players roll Initiative as normal, and add any modifiers. Any tie is broken by a d100. Players are then ranked in DESC order.

Then the highest non-player CR creature rolls Initiative.

If the highest CR creature rolls, if it scores beats the players, it goes first, if not, the player with the highest Initiative goes first. Ties are broken by the d100 roll.

Then based on the order of CR, the Initiative alternates between creature and player.

Any creature with a Summon id attached goes LAST in the Initiative count and is alternating.
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The main reason that this system Helps, is it dramatically Reduces variance in combat, and Helps out with combat Balance; mostly preventing lop sided Turns where the monsters or players go first and zerg a creature down.