Lord

The rank of Lord is the highest rank that can be attainable by anyone within the Death's Own and they're the most skilled operatives and agents within the cult. The Lords can be selected to teach initiates, followers and maybe even other lords if deemed skilled and experient enough, Through no one has been selected for that yet.

They're the only rank that is allowed to be sent to independently hunt Jedi.

Skills and Users
Lords are supposed to be in the minimum skilled in the arts of combat, assassination and stealth, They are supposed to also hold enough skill and knowledge to match a Jedi in combat without using the force. Most study the force abilities to a certain extent, As to be useful against those in combat.

Ari
Ari is a honor title that is given to those who manage to kill a Jedi or a particularly skilled Force User, It means "Lord" in Sith, And is one of the reasons that the Lords are the only ones sent to hunt trained/skilled Force Users. It is the highest honor one can achieve within the Own's and being a Ari means you're equal to the triumvirate in skill and talent, Giving you the same rights of one with the exception of making decisions and missions.