Tomashi 

Init 24

CON Save:  5

uncany Dodge reaction for half the damage

Trey's initiative: 5

Okay, Trey goes first.  You're fifteen feet away from them, but, being surprised, you can't move or use an action.  Any bonus actions?

OutcastFromAnotherSide, Pearlin is wandering around, lost, when she looks up to see a black-clothed figure dart lithely across a rooftop and leap to a window ledge on another building, landing soundlessly, before jumping inside.  A split second later, there is a yelp of pain from inside.

Also, GR, there is a person called "Balkur Singh" in the campaign who should not be there.  Can you kick them?

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Okay, Trey goes first.  You're fifteen feet away from them, but, being surprised, you can't move or use an action.  Any bonus actions?

On initiative Count 15: Trey will use Blessing of the Raven Queen to teleport to be adjacent to the intruder.

(But just wanna double check that I can. Because typically if you're unable to take actions you cannot take bonus actions also.)

It says, "you cannot move or take an action," not "actions," so yes.  However, it seems I was unclear - there is only one intruder, and I wrote "they" because you do not know their gender.

I'm sorry, I read GR's post wrong.  You were supposed to go first.  I thought that was the CON save.  After that it's Raiden, then the shinobi.

Bonus Action: Raiden's single tail splits into nine as he unleashes his katana, causing it to crackle with yellow lightning and bring his AC from 14 to 18. (Bladesong)

Not far from the ocean, a recently transformed siren hears the commotion and observes the ninja from above leaping inside. Usually she'd enjoyed watching the pleasure of others getting tormented by someone, but this time, she has an instinct to go after whoever that was. She's hoping as she struggles to get used to human legs for the first time that she's aiming for the right person and not the ones that needed to be assisted. "Oh my! Whatever in the sky beauties was that?!" 

Action: Dissonant Whispers 1st level. Ninja must make a DC 13 WIS saving throw, taking 14 psychic damage on a fail (and move further away from the mermaid herself) or half on a success

Initiative: 14

Okay, nice.  The ninja rolls a 15 on the Wis save.

If you'd like to climb up to the room, you'll need to make an Athletics check.  Tamachi can go, because I messed up the order a bit, then the ninja.

Tamashi will attack the ninja with his Soul Knife whip to attack and then back the other way for a BA attack:

Action: Attack: 19 Damage: 10

Bonus Action: Attack: 19 Damage: 5

If one of them hits, then Sneak Attack Damage: 12

Very nice.  The ninja stumbles forward as Pearlin's blast of sound hits them.  They seem a little dazed as Tamachi manifests a long purple whip of psionic energy, the gem dragon scale on his forehead glowing as he does so.  The ninja dodges the first lash, but Tamachi summons another in his off-hand and smacks the black-robed attacker across the head with it.  They clearly weren't expecting to face such staunch resistance, so they throw one more knife at Tamachi (15 to hit, 6 piercing damage on a hit, and you must make a DC 15 CON save, taking 19 posion on a fail, or half on a success) before disengaging as a bonus action and jumping back out the window.  They make it all the way across the street (which is 12 feet wide if anyone wants to follow), landing on the opposite rooftop and running 10 feet along it.  Back to Tamachi.

Trey would like to use his reaction to Opportunity Attack him. (Trey has Sentinel feat) He doesn't have a weapon drawn so it'll be unarmed fighting.

Attack: 8 Damage: 0

Damage: 4

If it hits his speed becomes 0.

That misses.

(That missed Tamashi)

As the ninja left, he took one last swipe at him:

Attack: 9 Damage: 9

(Presuming we are out of initiative order)

"What the #$%&^ was that? Any ideas why ninjas would be after any one of us?"

You aren't out of iniative, as you can chase them. (You should).