A Thousand Blazing Suns

A 5-star claymore with a CRIT Rate secondary stat. Using an Elemental Skill or Burst gives a short window of extra CRIT DMG and ATK, which can be triggered again after its cooldown. Normal or Charged Attacks that deal Elemental damage stretch that window, and in the Nightsoul's Blessing state the buff is stronger and does not run out while the wielder is off the field. That last part is why it is the signature weapon of Mavuika, but any claymore damage dealer with Elemental Normal Attacks can keep the buff up for much of a fight, which makes it a strong general pick.

Gain the "Scorching Brilliance" effect when using an Elemental Skill or Burst: CRIT DMG increased by 20% and ATK increased by 28% for 6s. This effect can trigger once every 10s. While a "Scorching Brilliance" instance is active, its duration is increased by 2s after Normal or Charged attacks deal Elemental DMG. This effect can trigger once every second, and the max duration increase is 6s. Additionally, when the equipping character is in the Nightsoul's Blessing state, "Scorching Brilliance" effects are increased by 75%, and its duration will not count down when the equipping character is off-field.