Maelstrom weapons are unique weapons that have a bonus which procs off a specific ability for each type of weapon (Bow, Melee, Staff). These weapons only drop from the final boss in veteran mode (one weapon per vMA completion). The drops are RNG based, with a chance to get any weapon in any trait each time you beat vMA. Each weapon is bound on pickup and drops in the Maelstrom motif style.
In case you need help with the Veteran Maelstrom Arena, make sure to check out the very in depth Maelstrom Arena Guide
, every stage and wave is explained in that guide. You can also find a lot of pictures that explain the mechanics of the Maelstrom Arena, especially the boss fights.