Death from below and Arcane Shift, does Arcane Shift mean I can't play Death from below anymore or can I retarget?

To clarify the interaction between Death from Below and Arcane Shift:

The Short Answer

You cannot use Arcane Shift to "save" a unit from Death from Below once Death from Below has been played. Because Arcane Shift is an [Action] speed spell, it can only be played when the game state is OPEN (no chain exists). Once your opponent plays Death from Below, it is placed on the chain, the state becomes CLOSED, and you are restricted to playing only [Reaction] speed cards.

Why this interaction works this way

1. State Requirements: According to Rule 309.1, if a chain exists, the game is in a CLOSED state. Per Rule 158.2.a, [Action] spells can only be played during an OPEN state.
2. Timing: When your opponent plays Death from Below, it enters the chain immediately. Because the chain is no longer empty, you cannot initiate a new chain with an [Action] spell like Arcane Shift.
3. Targeting: Even if you were in an OPEN state, Arcane Shift requires you to target both a friendly unit and an enemy unit at a battlefield to be played (as confirmed by RiftJudge FAQ #378 and #9022). You cannot play it solely to "blink" your unit if there is no valid enemy unit to target for the damage portion of the spell.

Can you "retarget" Death from Below?

No. Death from Below targets a unit when it is played. Once the spell is on the chain, the target is locked. Playing another spell (if you had a valid [Reaction] spell) would not allow you to change the target of Death from Below unless that specific spell explicitly says "change the target of a spell."

Summary of the Sequence

1. Opponent plays Death from Below: They choose a target unit at a battlefield. The spell is placed on the chain.
2. State becomes CLOSED: Because there is now an item on the chain, you cannot play [Action] spells.
3. Resolution: Unless you play a [Reaction] spell to counter or otherwise interact with Death from Below, it will resolve and attempt to kill the targeted unit.

This is my interpretation based on the rules for [Action] speed spells and chain states. No official FAQ exists for this specific interaction, but it is consistent with RiftJudge FAQ #9389 regarding Arcane Shift's inability to respond to spells already on the chain.