Well you have to upgrade it, but it will be pretty much a BiS I think. Maybe if you raid savage there will be a better one, but I think resistance weapons are really the best. One difference is that relic weapons don't get depowered. Well they don't have unique effects either, but still it is important to note that unlike Legion artifacts or the heart of azeroth, your relics will stay as strong as they are even if the new expansion comes up and you enter their zones. It will be outclassed for sure, but not immediately.
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Based on the penultimate cutscene with Zenos and Fandango and that roar, I have a fealing we already have Anima. It is the mysterious dreamer. We havent seen any Garlean princesses yet.