Complete "Hollowborn Lich King" farm → (See ITEMS / HOLLOWBORN channel)
Complete "Empowered Caladbolg + Dual" farm → (See BOOST % WEAPONS / 51 WEAPONS channel)
Place Altar of the Hollowborn Caladbolg in your house
→ Click it
→ Accept "Pay Homage to Caladbolg (Again)" quest
→ /join hbchallenge → Kill "The Darkdweller" in the last room
Complete the quest until you have both singular and dual "Hollowborn Caladbolg"
→ Head to /join Ultradage → Complete the merge