How can I unlock the Rune system?

The Rune system will be unlocked at level 15 of 2nd rebirth.
And there are 2 ways to open the Rune system.
a. Open from your character interface.
b. Open from your inventory.

Each Rune combination has 5 slots inside and 8 more slots in the outside which can be equipped with 1 Red Rune, 1 Blue Rune, 3 Yellow Runes and more. The Rune effect is determined by the combination being used.