Malichi heads southward from the main trunk on the last remaining branch at this height.  He slows as he approaches the end of the branch.  There are, indeed 3 more acorns out there, ripe and ready to be plucked like fruit from the vine.  However, something stands, or hangs rather, between the tiefling and his prize:

Malichi tries to sneak past the hornets nest.

Stealth: 9

So, that didn't work.  Three hornets exit the nest to investigate the disturbance.  Anyone who is present should post their actions.  The hornets appear hostile.

Zensiro had popped his head out of the AGGE to call upon assistance from Malichi.  With his keen powers of observation, he quickly notices Malichi stumble during his trek down the southern branch.  Further noticing the hostile hornets, Zensiro quickly draws his bow and nocks an arrow in case that hostility turns to outright aggression.

Malichi drops the grappling hook (it's still attached by the rope) and draws his bow

(Are Cerlinde, Dyumi, Zithers or Amelia participating in this encounter?)

(Sorry very busy lately!). Cerlinde hides in her seat in the AGGE, but ready to help with a fire bolt or spell.  (She’s terrified of getting stung)

The three vespids make a beeline for Malachi.  They do not appear open to friendly negotiation.  The tiefling and Zensiro, with Cerlinde as backup, are now in combat, please post your actions.

 Amelia will also join in.

(I think that Zensiro will have to use his Ready reaction to fire his longbow, then use BA to cast Hunter's Mark for next round's damage.)

Zensiro, sensing that the hornets have gone bad, unleashes his nocked arrow, hoping to protect Malichi from the vicious beasts.

Longbow - attack vs. Hornet 1: 26, damage (piercing): 12

Bonus Action - cast Hunter's Mark on Hornet 1 (or Hornet 2 if 1 is dead)

Cerlinde will target the same hornet as zensiro (unless it’s dead) with a fire bolt 

17 to hit

5 damage 

Malichi fires at hornet 3

shortbow attack: 21

damage: 6

Zensiro's readied arrow flies true, straight into the thorax of the lead hornet, delivering a near-fatal wound, which is made completely-fatal by the wizard's following bolt of arcane flame.  Malichi is also able to get off an attack, striking the second giant insect, before the remaining two foes converge on the tiefling.

H1 Attack: 10 Damage: 7 piercing + DC 11 CON save (8 poison damage on fail)

H2 Attack: 21 Damage: 6 piercing + DC 11 CON save (8 poison damage on fail)

(Need Amelia's Round 1 action)

Zensiro sees his friend Malichi under serious duress and focuses his aim on the successful assailant.

Bonus Action - refocus Hunter's Mark on the new H1

Longbow - attack vs. H1: 9, damage (piercing): 15 (w/Hunter's Mark)

(Sigh.)

Hearing the clamor, Dyumi will stop preparing Detect Poison and Disease to cast a Guiding Bolt from the safety of Aggie at the least injured hornet (assuming the fight is 120 ft or less away).

Attack: 12 Damage: Unable to parse dice roll.

Zensiro sends a follow-up arrow sailing into the great abyss, while Dyumi the druid disintegrates the H2 hornet from the comfort of AGGE.

CON: 7 

 Malichi takes 3 piercing damage, 9 poison damage and is poisoned.

Malichi tries another arrow at the closest hornet

Shortbow attack: 12 

Damage: 8

(Made a mistake on the attack roll, should ha e been +6 instead of +2)

The third hornet is defeated!  However, more hornetty noises are stirring within the hive.  Best gather ye acorns while ye may! (apologies to Robert Herrick)