You must complete the quest 'Juggernaut Items of Nulgath'. The process behind this is very complicated so it is best to examine the AQW wiki.
aqwwiki.wikidot.com/tercessuinotlim
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It is in the AC shop for 600 AC's.
You buy King Alteon's blade from the Good Shop (not good rep shop.) It's for members only sadly.
you get it from valsakar note:member only
The best aqw weapons/weapon is/are from nuglaths juggernaut items quest including crystal phoenix blade of nulgath, ungodly reavers of nulgath.overfiend blade of nulgath. Juggernaut items of nulgath is a quest that comes from nulgath in the area terccesuinotlim. Also dragonblade of nulgath(from demanding items of nulgath) is nice to. It comes from demanding items of nulgath. Ths quest comes from nulgath as well and can also be accesed from battle phoenix blades, sword of nulgath and betrayal blades of nulgath above the 7th betrayal blade
Bend over ill show you ;)