Goblin Warchief + Krenko, Mob Boss + Skirk Prospector

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Overview

Goblin Warchief reduces only generic mana in the cost of Goblin spells you cast [2018-04-27] and gives Goblins haste [702.10]. Skirk Prospector’s Sacrifice-a-Goblin-for- ability is a mana ability (it has no target and can produce mana) and can be activated while you’re casting spells to help pay costs [605.1a][601.2f][605.3a]. Krenko’s tap ability sets X equal to the number of Goblins you control as defined by its text; that X is determined on the stack and may change before resolution [107.3c], and it counts Krenko itself and all other Goblins you control [2012-07-01].

The StackRules & CitationsDetails & Edge Cases

Step 1

If you are casting Krenko, determine its total cost with Goblin Warchief’s reduction applied to the generic portion ( becomes ) as part of cost calculation [601.2e][2018-04-27].

Step 2

After the total cost is locked in and before you pay it, you may activate mana abilities to produce mana for the payment [601.2f][605.3a].

Step 3

You may activate Skirk Prospector’s Sacrifice-a-Goblin-for- because it is an activated mana ability that has no target and can produce mana [605.1a], and that ability resolves immediately after activation [605.3b].

Step 4

If you sacrifice Goblins (including newly created tokens) to Skirk Prospector before Krenko’s ability resolves, those sacrificed creatures are no longer counted as Goblins you control when evaluating X for Krenko’s ability [107.3c][2012-07-01].

Step 5

When you activate Krenko’s ability, the ability goes on the stack and X is the number of Goblins you control while that ability is on the stack; that value may change before resolution (so sacrifice timing matters) [107.3c].

Step 6

If Goblin Warchief is on the battlefield and a Goblin enters under your control this turn, that Goblin has haste from Warchief, letting it activate tap abilities or attack immediately as allowed by haste [702.10].

Resolution

When casting Krenko, Mob Boss you get the Goblin Warchief generic-cost reduction applied to Krenko’s generic portion ( → ) but Warchief does not reduce Krenko’s red pips () [2018-04-27][601.2e]. While you’re casting Krenko you may activate Skirk Prospector’s activated mana ability(s) to produce red mana because mana abilities can be activated before paying costs [601.2f][605.3a]; Skirk’s ability resolves immediately when activated because it’s a mana ability [605.3b]. If you activate Krenko’s ability, X is the number of Goblins you control as that ability exists on the stack and may change before it resolves [107.3c], so any Goblins you sacrifice (including ones you sacrifice to Skirk Prospector) before Krenko’s ability resolves will reduce X [2012-07-01]. You may sacrifice Skirk Prospector itself to pay for mana, but do so last if you need to sacrifice multiple Goblins because once Prospector is sacrificed you can’t activate it again [2022-12-08].