This occurs once you reach Infinity Infinity Points.

Eternity is the 4th consecutive reset system, which resets everything except your achievements, statistics, challenge times, Infinity Challenge times, Time Dimensions, Eternity Upgrades, Time Studies, Eternity Milestones, and Eternity Challenges. This can change depending on which Eternity Milestone you have reached.

What can I do now? Edit

Unlike Infinities, you can exceed Infinity IP whenever you want, and you will gain more EP as a result of doing so. The formula for EP is

EP = floor(5^(floor(log10(IP))/308-0.7)*bonuses) ~= 0.324*bonuses * IP^(1/440.65)

With no bonuses, the amount of IP needed for various amounts of EP for the purposes of early post-eternity is listed below:

EP IP Needed
1 1.8e308
2 1e349
3 1e426
4 1e481
5 1e524
6 1e559
7 1e588
8 1e614
9 1e637
10 1e657

There is a more extensive spreadsheet of IP -> EP values available here.

EP vs IP

EP and the exponent of IP needed to get them

You able to buy time theorems how far you go and buy more upgrades which are called Time Studies. You also unlock new set of dimensions called Time Dimensions which give you time shards which translated to free tickspeed upgrades.

What's next? Edit

After reaching enough time theorems, you will able to do Eternity Challenges. This will let you reach Infinity EP but you will get an achievement not a new prestige layer.