Temple of Eternals
At the Temple of Eternals you can prestige to obtain firestones, which increase your gold earnings.
Prestige
When you earn gold on the battlefield you will also get firestones, but you need to prestige to actually collect them. You have to gain 6 times as much gold to double the amount of newly collected firestones:
firestones = (gold / 18000) (log 2 / log 6) ≈ (gold / 18000) 0.38685
The amount of firestones you get can be increased with the Prestigious research.
When you click the "Choose Prestige" button, you can choose between normal prestige and epic prestige. Epic prestige doubles your firestone gain for the current run for the price of 1 prestige token or 200 gems. When you prestige, you lose all your current gold and gold upgrades and start the game over at stage 1 (but you keep everything else).
You will automatically get a notification to prestige whenever you have a ×2 multiplier. This means that the amount of firestones that you have doubles when you prestige. The newly collected firestones will be added on top of the old firestones.