Tona's Flame in Genshin Impact is the new offering system found in Natlan. Luckily, unlocking it is very simple. Similar to the offering systems in other in-game regions, you can offer Pyro Sigils to obtain several rewards, including Primogems, Acquaint Fates, and Natlan Shrine of Depths Keys.
This article lists all the rewards that can be obtained from the new Tona's Flame offering system.
Genshin Impact Tona's Flame location in Natlan and rewards
Tona's Flame is Natlan's new offering system introduced in Genshin Impact 5.0. You can unlock it just by approaching it but finding it can be a little difficult since it is located underground.
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Start by teleporting to the eastern waypoint in the Stadium of the Sacred Flames and head straight before turning left in front of the main stadium entrance to find a door on your right. Enter it and head deeper into the building before going downstairs to reach the blacksmith NPC. Finally, take two more right turns to find the Tona's Flame.
Unfortunately, there's a level cap on the Tona's Flame offering system, and you can obtain the rewards up to level 20. That said, you can keep opening chests to collect Pyro Sigils and use them once the restriction is removed.
It is also worth adding that while you cannot go over level 30, you can still view the rewards up to level 35: