Effect
Astral Presence
Astral Presence is a plain resource passive: it raises your maximum Arcane Power and speeds up its regeneration per second. This gives you more reserve for expensive spenders and refills it faster between casts, an unobtrusive but noticeable cushion against resource shortages.
The slot is most valuable in builds that regularly fire Arcane Power-hungry spenders like Arcane Orb or Black Hole and would otherwise stall without extra regeneration. Because the boost is constant and independent of combat, the passive is especially reliable when your resource flow would otherwise fluctuate. In playstyles that already cover their Arcane Power through other sources, such as cheap runes or generator-based recovery, the benefit is smaller and the slot is better spent on a damage or defensive passive. It is, therefore, a dependable foundation pick for spender-heavy setups.
What the two values mean
The first part raises your maximum Arcane Power by 20, the second adds 2.5 Arcane Power regeneration per second. The larger reserve helps at the start of a fight when you cast several spells back to back; the regeneration pays off in long fights where the bar is drained without a break.
Astral Presence unlocks at level 24, right when the first expensive spells arrive on your bar while leveling.
Who benefits from the Arcane Power
Energy Twister costs Arcane Power with every cast, making it a typical spender that lives off faster refills. Magic Weapon and Storm Armor each cost 25 Arcane Power to activate, and with the larger reserve you do not start the next fight with a half-empty bar after refreshing them. Teleport, by contrast, runs on a cooldown and is unaffected by the passive.
Three passives, three jobs
Astral Presence often sits next to Audacity and Galvanizing Ward. The split is clear: Audacity provides damage against nearby enemies, Galvanizing Ward a shield after a while without taking damage, and Astral Presence the resource. If you notice your Arcane Power never runs short, you can free the slot for another damage passive.