Devarus lingered around long enough for the initial exchange; but, after a bit of a chuckle and shake of the head, he follows after Val to test out his new fishing pole, if nothing else! Although, if at all possible during the excursion, he keeps an eye on the sparring.

“I’ve learned a few new tricks, in fact, I’ve got a new one I’d love to try out.” She then motions toward Runt and says, “Have you ever seen this one fight? She’s a force to be reckoned with.”

Kanai looks at Runt and gets excited. He says, "Really? This one is that strong? This is perfect! We can have a 2 against 1 like last time." He turns to Runt and says, "Hey! You over there! I don't know you're name but my Rival said you're really strong. Do you want to spar?"

Runt grins and approaches. Only if my battle sister wishes me to join your duel.

Kanai turns and give Gendan the puppy-dog eyes. Looks like he wants you to say "yes" to Runt joining in on the duel.

“Sure, lets do this. Kanai, you may want Val on standby for healing again.” She gives a little chuckle.

Roll initiative

Kanai's Initiative: 8

18

4

Gendan's turn.

Gendan casts Sword of Ravengar, bringing a new blade to her arsenal, and approaches her foe. Then Attacks 9 for 15 magical slashing. Then she swings it again 25 for 12 magical slashing. 

EDIT: it should be force damage, not magical slashing. 

First attack missed but the second hit.

DM: Northwest, Sword of Ravengar is a warlock spell and you can have up to 6 Warlock spells prepared. You currently have 7. Please remove one spell to make room for the sword.

Kanai Counter attacks: 27 Dmg: 7
Kanai Counter attacks 2: 24 Dmg: 6 Trip attack extra damage: 3 STR Save: 19 (DC17)
Gendan falls prone.
Flurry of Blows 1: 19 (Adv:17) Dmg: 10
Flurry of Blows 2: 27 (Adv:16) Dmg: 9

Stunning Strike on 2nd normal attack: Save 5 (DC14)

Kanai is awestruck at the sword that materializes in Gendan's hand. He is taken quickly out of his stupor as Gendan attacks. The monk bends back to avoid the first attack but can't dodge the second attack. He falls to the ground but he puts his feet together and jumps forward with a quick to Gendan. She quickly raises her shield to block it but Kanai is quick on his feet and with an acrobatic motion, he jumps off the shield and lands behind the warlock. As Gendan turns, Kanai's round-house kick, lands on her face and knocks her down to the ground. Kanai attempts a two handed smash while she's on the ground but Gendan rolls out of the way in swiftly.

Runt and Gendan's turn.

(A ruling question regarding the home brew AoO rule: Would a prone creature gain an AoO if an adjacent prone creature tries to stand?

Also, I assume if Runt were to pincer Kanai, it would nullify the disadvantage for attacking from prone for Gendan, right?)

Yes. A prone creature will trigger AoO from another prone creature but the AoO would be made with disadvantage. And pincering will cancel disadvantage from attacking from prone.

Elsara will keep a watch from the carriage, more wanting to spend time with the lizards because she thinks they're cool (not as cool as Twinkle Snow), but keep a view from a distance to make sure there's no foul play in the sparring match.

After watching the acrobatic display, Runt activates her super power and gives a raging battle cry before swinging at the monk with the flat of her axe.

Attack: 24 Damage: 13

Attack: 20 Damage: 14

Both attacks hit.

Attack Vs Runt: 16 Dmg: 8
Attack 2 Vs Runt: 16 Dmg: 10

FoB Vs Runt: Attack Vs Runt: 19 Dmg: 6
Stunning Strike CON 20 Vs DC14

2nd FoB vs Gendan: 29 (Adv: 15) Dmg: 11
Crit: 6
Stunning Strike: 8 Vs DC14 CON

EDIT: I messed up. I skipped Gendan's turn on accident. So now... Gendan gets 2 turns and Runt gets one turn this round. Since she is stunned, Gendan will get one attack off before being stunned on the 2nd turn.

So the turn order for next round is:

Gendan
Gendan (Stunned)
Runt
Kanai

Runt's battle cry startles Kanai long enough to get two very solid hits on the monk's head. The monk counters by two swift punches aimed at Runt's face but Runt manages to dodge them. Then she is hit between the eyes by an attack that was meant to stun her but she managed to tank it.

Gendan's Unstunned turn.

Gendan crawls around Kanai to be opposite Runt, then Hexes Kanai (DEX). Then Gendan uses the “Shove a creature” action to try to yank Kanai’s feet to make him prone with her. Athletics check 16, Kanai must make an Athletics check or an Acrobatics check at Disadvantage to avoid going prone.

Runt should get her turn (Round 2, with advantage from pincering) before Gendan uses her “stunned” turn (beginning of Round 3).