Fused Leapstones are a fresh consumable item in Lost Ark, which was recently added in the May update. Considering how high your gear is, the more resources you may have to invest in upgrading. Nevertheless, Fused Leapstones simply give you a free chance to upgrade your gear. If you get lucky, you essentially get a free upgrade.
Currently, Fused Leapstones are only available to players who have hit Item Level 1415 and above. These stones are dropped as a reward from Demon Beast Commander Valtan, the powerful Legion Raid that requires an entire team and lots of coordination. This is why you most likely wont be seeing these Leapstones anytime soon.
Fused Leapstones help you achieve a guaranteed upgrade, thus you will not be able to use any protection materials to increase your chances of an upgrade. In addition, you only have one flat chance to successfully level up your gear.
This makes it difficult to fully utilize Fused Leapstones in upgrading your gear. These items should be seen as brief attempts to get a free upgrade on your gear, but do not expect too much. The probability for upgrading gear was as low as 10%, making it a rare event where the Fused Leapstones would result in an actual upgrade. However, if it has been successful, then you have just saved a ton of resources.
Fused Leapstones in Lost Ark are all you need to know. Participate in your Legion Raids and cross your fingers for some excellent wealth from Lady Luck.