There are a variety of activities to do in Horizon: Forbidden West. This page will concentrate on how to unlock the Arena.
The Arena is an activity that will pit you against various Machines in a controlled environment. In order to unlock the Arena, you will need to have progressed the story missions until you reach a three-way split. The Broken Sky will be the lowest leveled mission of the three, which is the one you want. Finish that one to open up a new story mission, called The Kulrut, then complete that one to finally set yourself on the right path to opening the Arena.
How to Unlock the Arena
Upon completing The Kulrut main story mission, you will find that The Maw of the Arena side of the Memorial Grove is now open. Fast travel to the Campfire right outside of the building and the entrance to the Arena portion will be directly west of it. Once inside, seek out Kalla and Dukkah, who will be marked with a green exclamation point, like any other normal side quest in the game. While it doesn’t really seem like a normal side quest and more of an Errand, either way, you will have Opening the Arena in your quest log.
In order to reopen the Arena, Chief Hekarro wants the ballista protecting the field repaired, which needs some machine parts. That is where you come in, as they both want you to retrieve those parts. Specifically, you will need two Bristleback Tusks and a Sinew from a Rollerback. The Tusks are something you may have already, since you can get them anytime. If not, then there is a Fire Bristleback site just north of the Campfire right outside the Arena. For the Sinew, the Rollerback doesn’t have this item normally, so you will actually have to hunt this one down to get the quest item from it. The Rollerback was said to have been spotted by a campfire to the southeast (as well as some Bristlebacks, according to Kalla).
Head to the location marked on your map, which will be a fair ways to the southeast of the Arena. Once you arrive at the location, you will see the Rollerback, but also some other machines, probably, like a Spikesnout and some Burrowers. If you can get some Silent Strikes on the Burrowers, that will help thin the numbers down a little bit, but that still leaves the Spikesnout and your target, the Rollerback. There are a lot of rocks in the area you can use for cover, but make sure to watch out for the rolling attack, as well as projectiles that can build up to inflict the Adhesive status effect. Once you defeat the Rollerback, loot it to get the Sinew and bring it back to Kalla at the Arena, along with the two Tusks, to finish and reopen the Arena for good.