How does Hextech Anomaly work, specifically how to use it to convert power into energy?

Question

Accepted Answer

You may exhaust Hextech Anomaly and pay any amount of Power (rainbow runes) from your Rune Pool to add that much Energy to your Rune Pool. You can activate this Reaction during the Pay Costs step (or any time the game directs you to add resources to pay costs), and the added Energy enters the Rune Pool immediately so it can be spent to pay the cost you were working on.

Activate Hextech Anomaly's Reaction by exhausting it and paying the desired amount of Power from your Rune Pool. The ability resolves immediately and adds that many Energy to your Rune Pool, which you may then spend to pay costs (for example, the Energy cost of the card or ability you were finalizing). This use is allowed during Pay Costs per the core rules. The card's parenthetical statement "(Abilities that add resources can't be reacted to.)" is ambiguous relative to the core rules; see assumptions.

Decide to play a card or finalize an ability that requires Energy, but you lack sufficient Energy and instead have Power in your Rune Pool. Begin the play process and reach step 4, Pay the card's costs (this is the moment you may use Reaction-tag Add abilities to add resources) [357]. Activate Hextech Anomaly's Reaction as an activated ability: exhaust Hextech Anomaly (its activation cost) and declare the amount of rainbow Power you will pay to the ability's cost. Pay that amount of Power from your Rune Pool to the activation cost of Hextech Anomaly. The Add resolves immediately and adds that many Energy to your Rune Pool [429] [165]. Spend the newly added Energy immediately to pay the Energy portion of the original card/ability's cost (since you're still in the Pay Costs step) [357] [429].

You cannot activate Hextech Anomaly if it is already exhausted (exhaust is a cost). The Power you pay must already be in your Rune Pool; you can't retroactively use Energy to pay the Power cost of the Reaction. The Energy added by the Reaction resolves immediately and cannot be responded to or interleaved with further reactions; priority/focus will not pass during that finalization [429.2.a] [429.3.a]. You cannot use the Energy added by Hextech Anomaly to pay the activation cost of the same activation if that activation required Energy as a cost (because the activation's costs must be paid to activate it). In Hextech Anomaly's case, the Reaction's cost is exhausting the gear and paying Power, so the added Energy is available after the activation finalizes. The card's parenthetical "(Abilities that add resources can't be reacted to.)" conflicts with the explicit rule allowing Reaction-tag Add abilities during Pay Costs [429.3]. See assumptions for how this ruling treats that text. If an effect restricts what Energy can be spent on (e.g., "Spend this Energy only during showdowns" on other cards like Diana - Scorn of the Moon), that restriction still applies to any Energy in your Rune Pool, including Energy added by Hextech Anomaly.

What Hextech Anomaly does and how to use it

Hextech Anomaly has a Reaction activated ability that reads (paraphrased): exhaust it and pay any amount of rainbow Power to Add that much Energy. To use it you exhaust the gear and pay the Power from your Rune Pool; the ability then Adds the same amount of Energy to your Rune Pool.

Why you can use it while paying costs

The play process' Pay Costs step explicitly allows a player to use activated abilities with the Reaction tag that Add resources to add Energy and Power to pay costs [357]. The Add rule further explains that activated Add abilities with the Reaction tag can be activated during the playing or resolution of other spells and abilities any time those spells/abilities require resources be paid, and that an Add activated in this way finalizes and resolves immediately [429] [429]. Because of this, you can activate Hextech Anomaly during step 4 (Pay Costs) to convert Power you already have in your Rune Pool into Energy that you can immediately spend on the cost you are finalizing [357] [429] [165].

Timing and immediacy

When you activate Hextech Anomaly in such a window, the Add finalizes and resolves immediately and priority/focus does not pass while it is being finalized or resolving [429.2.a] [429.3.a]. That means you won't get an opportunity for other reactions to interleave with the actual Add—once you pay the Power and exhaust the gear, the Energy appears in your Rune Pool right away.

Practical example

You want to play a spell that costs 4 Energy but you only have 2 Energy and 2 rainbow Power in your Rune Pool. During Pay Costs [357], activate Hextech Anomaly: exhaust it and pay 2 rainbow Power. The Add resolves immediately and adds 2 Energy to your Rune Pool [429] [165]. You now have 4 Energy total and can spend it to pay the spell's Energy cost [357].

Notes about the card text parenthetical

The printed parenthetical on Hextech Anomaly says "(Abilities that add resources can't be reacted to.)" This is ambiguous when read with the provided rules, because the rules specifically permit activated, Reaction-tag Add abilities to be used during Pay Costs [429.3]. The most defensible interpretation consistent with the rules is that the parenthetical is a reminder that Add abilities finalize and resolve immediately (so you will not get further opportunities to interleave other chain items with the Add), rather than a statement preventing you from activating the Reaction to add resources when allowed by the rules. See assumptions for an explicit statement about this ambiguity.

Relevant rules cited: Pay Costs allows Reaction Add activations during step 4 [357]. Adding puts resources into your Rune Pool and players must add before spending them [165]. All Add abilities add resources [167]. Activated Reaction Add abilities can be used during Pay Costs and resolve immediately; Add finalizations don't pass priority and resolve before other pending chain finalizations [429].