Mythic upgrade Stones are used to upgrading Mythic Gear to Mythic Tier 1 (T1) and Mythic Tier 2 (T2). You need T1 stone to upgrade Mythic gear to Mythic Tier 1 and T2 stone to upgrade Mythic Tier 1 to Mythic Tier 2.
To upgrade your Mythic item you will need 1 Stone per one item, and Stone will be consumed in the process. To use the Stone select the item you want to upgrade and choose the Strengthen button.
Mythic upgrade Tier 1 Stones will start to appear in your AFK rewards chest after beating stage 20-60, Mythic upgrade Tier 2 Stones will start to appear in your AFK rewards chest after beating stage 25-60. They will also appear in both Store and Guild Store available to buy for Diamonds and Guild coins respectively.
You can also obtain the Tier 1 Stones from King's Tower Rewards for completing the quest to beat certain floors (100, 120, 140, 160 and 180), as rewards in Peaks of Time or buy them in Special offers.