Echoes of Eternity + Skittering Invasion + Ulalek, Fused Atrocity

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Overview

When you cast Skittering Invasion with three Echoes of Eternity on the battlefield, you get nine Echoes copy triggers and four Ulalek triggers. If you resolve all nine Echoes copy triggers first (each creates one copy of the sorcery), then resolve each of the four Ulalek triggers and pay for each, each Ulalek doubles the number of Skittering spells on the stack; starting from the original + 9 Echoes copies = 10 spells, four paid Ulalek resolutions produce 10 * 2^4 = 160 Skittering spells total, which each make five Spawns for 160 * 5 = 800 Spawn tokens. This behavior follows casting-trigger timing [601.2h], the rules for copying spells and abilities [706.10], and how copying triggered abilities that refer back to the original trigger can be undefined [706.7].

The StackRules & CitationsDetails & Edge Cases

Step 1

You cast Skittering Invasion. Because you cast a spell, any abilities that trigger when you cast a spell trigger now [601.2h].

Step 2

The triggers produced at that moment are: one 'Whenever you cast a colorless spell, copy it' trigger from each Echoes of Eternity (3 originals), and Ulalek's 'Whenever you cast an Eldrazi spell...' trigger (1 original).

Step 3

Each Echoes permanent’s static line saying 'If a triggered ability of a colorless spell you control or another colorless permanent you control triggers, that ability triggers an additional time' causes each triggered ability that qualifies to trigger extra times for each other Echoes on the battlefield. With three Echoes the math is: each Echoes' own copy trigger is duplicated by the two other Echoes, so each Echoes yields 3 trigger instances (original + 2 additions) -> 3 * 3 = 9 Echoes-copy triggers in total. Ulalek’s single trigger is duplicated by all three Echoes -> 1 + 3 = 4 Ulalek triggers.

Step 4

All 9 Echoes-copy triggers and the 4 Ulalek triggers exist on the stack together; you, as controller, order them as you choose when they’re placed on the stack [601.2h].

Step 5

If you resolve all 9 Echoes-copy triggers before resolving any Ulalek trigger: each Echoes trigger, when it resolves, makes a copy of the Skittering Invasion spell and puts that copy onto the stack [706.10]. After resolving all 9, there are the original Skittering plus 9 copies = 10 Skittering spells on the stack.

Step 6

Now resolve an Ulalek trigger and pay when it resolves. Ulalek's effect copies all spells you control (it copies the original + the 9 copies = 10), putting 10 more Skittering spell copies onto the stack [706.10]. If you resolve the remaining three Ulalek triggers and pay for each one in turn, each paid Ulalek will copy all spells you control at that moment, effectively doubling the total number of Skittering spells each time that Ulalek resolves.

Step 7

Concretely: start 10 spells after Echoes resolved; after first paid Ulalek -> 20 spells total; after second paid Ulalek -> 40; after third -> 80; after fourth -> 160 Skittering spells total. Each Skittering spell on resolution creates five 0/1 Spawn tokens, giving 160 * 5 = 800 Spawn tokens, assuming you pay for all four Ulalek triggers and no other effects intervene.

Resolution

Casting Skittering Invasion with three Echoes of Eternity causes nine Echoes copy triggers and four Ulalek triggers to be created; by resolving all Echoes triggers first (creating 9 copies) and then resolving each Ulalek trigger and paying each time, you end up with 160 copies of Skittering Invasion (original + 159 copies) and therefore 160 * 5 = 800 Spawn tokens. This follows the rules for triggers on casting [601.2h], copying spells and abilities [706.10], and the interaction that copies of triggered abilities made later lose the original trigger context and may do nothing for references to that original event [706.7].