In Elden Ring, a weapon upgraded with Somber Smithing Stones reaches a maximum level of +10.
Somber Smithing Stones improve unique weapons, including many weapons obtained from bosses, special chests, and important NPCs. These weapons use a shorter upgrade path than ordinary weapons, which are improved with regular Smithing Stones.
The final upgrade requires an Ancient Dragon Somber Smithing Stone. These rare materials are limited in a normal playthrough, so players usually reserve them for weapons they intend to use for a large part of the game.
The most common mistake is applying the +25 cap for standard weapons to unique weapons. Standard weapons can reach +25, while somber weapons stop at +10; the two scales are different but represent comparable endgame enhancement.