The One Ring is a concept, where all books and tier runes are pumped into one good relic. Basically an "ultimate weapon", as this was the books don't go "waste", scattered around all equipped items.
tl;dr version is: have a Legendary item level 20+ jewelry item (ring/necklace). Then use the forge to make it into a Relic with 80+ to all six of the stats. If it is under 80, you can increase it with books. Then use Tier Upgrade Runes to increase it to +7. Now you have a "One Ring" (or necklace) that you can continue to book without worrying that you are wasting books.
When you upgrade a jewelry item with a Tier Upgrade Rune, it will receive an increase in each of its six stats according to whichever one is lower:
- 25% increase (rounded down) from its current stat
- its item level
This is why you want an item level of at least 20, and stats of at least 80. However, you will probably benefit more early on if you use this on an item level 20 Relic that might not have 80 in everything. For example, 64 in every stat means you will only miss out on 4 stats to everything as the end product after using Tier Upgrade Runes (because after the first Tier Upgrade Rune used, the 64 -> 80, and you are on track for the other +6 upgrades). So I recommend that you dont get obsessed with the amount of stats or how close you are to 80, just make sure you have a Relic with item level 20. Of course if you rolled garbage stats on your Relic, you may want to reroll or choose another Relic instead.