Tearblaster is a secret hidden weapon in Horizon Zero Dawn. Here's how to get it:

1) Complete Errand "Hunting for the Lodge". It triggers by getting 3 Half Sun Marks at Hunting Ground Trials.

2) Complete follow-up side quest "Hunter's Blind". It triggers automatically after Hunting for the Lodge, mark it in your quest log.

3) After Hunter's Blind you get a Tearblaster treasure box as a mission reward.

4) Go to inventory - treasure boxes - tearblaster weapon box - open it and equip under weapons.

Even though this quest line isn't very secret, the weapon can easily be overlooked among the many other reward boxes. It's a good weapon for early to mid game. What it does is to shoot compressed air, thus tearing off machine components / armor / heavy weapons in one hit. Use it against enemies that carry weapons on their backs, then use their own weapons against them (especially the big Thunderjaws are very easy to kill this way). It's also good against flying enemies to knock them out of the air or to shoot machine components with fluids that will then explode. The tearblaster stuns most enemies in a few hits. They then fall to the ground for a few seconds where you can follow up with a critical spear hit. The range is very limited, comparable to a sawed-off shotgun.

And that's how you obtain the Tearblaster in HZD. Enjoy your new weapon!