Thri-Kreen really have a lot going for them. For starters, they’re monstrosities. Charm person, hold person, and dominate person are all out the window. Next, their carapaces give them an unarmored AC of 13 + their DEX mod. That same carapace changes color and texture to give them advantage on stealth checks. They’re telepathic, and they’ve also got dark vision, and don’t need to sleep.
If that weren’t enough on top of getting a generous ability score bump, they also get a second set of arms. This set of arms can hold light weapons, manipulate objects, open doors and wield light weapons. What good is that though? Outside of combat, it’s fun for roleplay, and you may be able to get your DM to give you advantage on certain skill checks.
Thri-Kreen in Combat
But what about inside of the action economy of combat? Is there any way to take advantage of those extra arms? Drawing or sheathing a weapon is a free action, but 5e rule only allow you to interact with one object for free each turn. As a result, players often choose to drop the weapon they are holding in order to draw and use another weapon. This, of course, would allow an enemy combatant to scoop up your weapon and use it against you. Maybe don’t drop that vorpal sword.
Can those extra arms do anything to help out in the situation? I don’t think they can. If the text had been written to allow a second free action to be taken with those arms, then yes. You could drop a sword and catch it with your secondary arms while you used your primary arms to draw and fire a bow. Sadly, it looks like having a second set of arms doesn’t let you do more in six seconds.
Now, two weapons fighting, though, we might have something here. Two weapon fighting allows you to use two light weapons, and make an attack with the second weapon as a bonus action. The wording of the rule here presumes that you have two hands. I’d really like to find a way to squeeze a larger weapon into the mix here. Sadly, I’m not seeing how to get there. What we can slide in is a shield, and maybe a hand crossbow. Try this loadout on something like a swashbuckler rogue. You can get a little AC Bump that you wouldn’t normally get, and some good ranged and melee attack options.