Mystic Forge + Sensei's Divining Top + Ugin, the Ineffable

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Overview

Sensei's Divining Top can rearrange or put itself on top of your library to set up what Mystic Forge sees and lets you cast; Mystic Forge lets you look at the top card any time and allows you to cast artifact spells and colorless spells from the top of your library. Ugin, the Ineffable’s +1 exiles the top card of your library face down (you look at it) and creates a linked Spirit token; that exiled card is no longer in your library so Mystic Forge can’t let you cast it from there while it’s exiled [406.3] and the static cost-reduction from Ugin applies normally when you cast colorless spells from the top of your library [601.2e] [601.2].

The StackRules & CitationsDetails & Edge Cases

Step 1

Set up: With Mystic Forge and Sensei's Divining Top on the battlefield, use Top's ability to look at the top three cards and reorder them so the desired artifact or colorless spell is on top of your library.

Step 2

Casting from the top: If the top card is an artifact or a colorless spell, Mystic Forge's static permission allows you to cast that card from the top of your library, and you begin casting following the steps in [601.2]; Ugin’s static cost reduction ("Colorless spells you cast cost less to cast") is applied when determining the total cost in step [601.2e].

Step 3

During casting the top card moves from library to the stack as part of 601.2a, so the new top card (if any) won’t be revealed or able to be looked at until the spell becomes cast per [401.6] and [601.2h].

Step 4

If instead you activate Ugin’s +1 to exile the top card of your library face down, that card leaves the library and becomes an exiled face-down card. You look at it (Ugin’s instruction), and per [406.3] you may look at that exiled face-down card again for as long as it remains exiled, but Mystic Forge cannot cause you to cast that card from the library while it is in exile [406.3].

Step 5

If the exiled card is linked to the Spirit token created by Ugin’s +1, the token leaving the battlefield will cause that exiled card to be put into your hand (per Ugin’s ability text); only then would you be able to cast it from hand (or otherwise play it) unless another effect specifically allows playing from exile.

Resolution

You may use Sensei’s Divining Top to see and reorder the top cards of your library (or draw and put Top on top) to present a desired card to Mystic Forge, and Mystic Forge will allow you to cast that top card if it is an artifact or a colorless spell from the top of your library. If Ugin’s +1 is used to exile the top card face down, that card leaves the library and is in exile, so Mystic Forge no longer applies to it while it’s exiled (see [406.3]). Ugin’s static ability that makes colorless spells you cast cost less applies when determining total cost as part of casting that spell [601.2e] and is locked in as normal during casting [601.2].