Abathax has True name in his hand and moves forward.

He starts by casting a 2nd spell as a Bonus action to challenge Merico to fight him; DC16 Charisma save. After which he attacks, Spellblade to use one Illrigger spell and one attack:

With Vengeful blade he brandishes his weapon and than strikes:
Attack: 16 Damage: Unable to parse dice roll.

On a hit, the target suffers the weapon attack’s normal effects and then radiates a dark aura of energy until the start of your next turn. If the target makes an attack or casts a spell before then, the target takes 2d8 Necrotic damage and the spell ends.

After which a normal strike follows:
Attack: 13 Damage: Unable to parse dice roll.

On the first hit he places a seal on Merico; can't be seen by others, but for Abathax it shows up as a glowing glyph.

Infernal Challenge: "You offer a creature a compelling challenge. If you have no allies within 5 feet of you, choose one creature within range who can see and hear you. They must succeed on a Charisma saving throw or answer your challenge and fight you. For the duration, you gain a +2 bonus to AC, the target has disadvantage on attack rolls against creatures other than you, and the first time the target tries to move away from you on a turn, they must succeed on a Charisma saving throw or their speed becomes 0 until the start of their next turn.

This spell ends if you end your turn more than 30 feet away from the target."

Retry damage rolls: 10 

12 

Abishai follows Abathax, moves down into the pit's southeastern area, and sends a whisp of energy at Merico.

Attack 25

Damage 11

He then hums a tune that inspires Wendell, giving the caster a 1d8 to use as he chooses.

Merico's DC 16 Charisma Save against Infernal Challenge: 25
Concentration Checks, DC 10: 9,  5,  7

Merico grits his teeth and grunts in pain as Abathax successfully slashes at him with his blade twice, before then being struck by a starry wisp from the descending Abishai.

Wendell is up!

[Wow not a single success on the concentration saves lol, one failure would be enough to break it but surprised to see not even a single pass!]

Abathax's first strike of his blade is enough to shatter the concentration of the old warlock, who howls in pain and despair as the portal closes in a swirl of mist. However, he resists Abathax's spell. 

Three rounds before Qualksander arrives.

Initiative
Abathax: 23
Abishai: 19
Wendell: 19
Merico: 18, -33 HP arcane gate expended, Vengeful Blade effect
Silvosh: 8

I was thinking it's a shame I didn't learn dispel magic, but it wasn't needed.

Wendell runs down to get sight and casts mind spike at 3rd level (gaining back his 2nd level slot from casting knock due to divination spell casting).

DC15 wis save or 19 psychic damage and his location is always known to me (with concentration for an hour). Half damage on a save.