Archon Shards
Feel free to put some shards in there for example two yellow shards for casting speed (or just one tau-forged shard), two blue shards for extra energy (because why not lol), and yeah some red shards for extra strength if desired.

Arcanes
You can put any arcane you desire in the second slot. Arcanes like molt augmented, molt efficiency, arcane acceleration, arcane tempo, and arcane avenger would be good options (I usually put either molt augmented for extra charges on Mesmer Skin or arcane avenger for that juicy 45% flat critical chance increase on all weapons)

Helminth Abilities
You can put any helminth ability. I suggest putting Roar or Eclipse or even something like Tharros Strike. I prefer having roar since it allows revenant to one-shot non-enthralled enemies who have at least 2 stacks of viral. Please refer to the build by Ninjase for a better understanding of how Reave works (https://overframe.gg/build/374539/revenant-prime/the-lich-king-revenant-general-use-endurance-steel-path-mesmer-tank-and-one-shot-reave/).