The Diablo 4 Brought to Heel side quest asks you to collect a red mushroom for Lumir, an NPC you find once you've explored the south and ventured into Hawezar. This is a fairly high-level area so you won't normally come here until later. Even if you decide to farm renown by uncovering every portion of the map, you won't be able to see any side quests until you reach the appropriate level.
If you're still low-level, knowing the best XP farm spots could help you out—you'll also want to get your mount unlocked as soon as possible to make getting around a little easier. For everyone else, here's how to find the red mushroom so you can complete the Diablo 4 Brought to Heel quest.
Where to collect a red mushroom for Lumir
The Brought to Heel quest is found in the town of Backwater, on the eastern coast of Hawezar. The place is pretty easy to miss as it's fairly tucked away, but you'll find a waypoint there, as well as the other regular town NPCs. The Brought to Heel quest is found at the very north of the town.
Once you speak to Lumir, he'll ask you to collect a red mushroom and bring it back to him. The area outside Backwater containing the mushroom is highlighted on the map but as it's quite large, and red mushrooms are relatively small, you've got a lot of ground to cover to find one.
It seems that the mushrooms can spawn anywhere inside the marked area, and a lot of the difficulty comes from not knowing what to look for. They're not brightly coloured so are pretty difficult to see as they blend in well with the rest of the ground. Look for fairly large patches of reddish-coloured circular growths, and hover your mouse over anything that looks suspicious—you can check the screenshot above to see what you're looking for. I found mine in the bottom-left corner of the area, though you might find yours elsewhere.
Once you've collected the red mushroom, head back to Lumir at Backwater and hand it over. Congratulations, you've just completed the Brought to Heal quest which rewards you with a rare item, a chunk of XP, and that all-important renown.