If the target for Death from Below is killed before resolution, can I still use the recast effect?

To determine if you can use the "recast" effect of Death from Below if its target is killed before resolution, we must look at how the game handles illegal targets and information checks.

Ruling

No, you cannot use the recast effect.

According to Rule 359.3.e.12, if a spell checks information about a target that is no longer legal (such as its Might), that check returns "null" and all calculations based on it are ignored.

Sequence of Events

1. Play: You play Death from Below, choosing a target unit at a battlefield.
2. Response: An opponent plays a reaction that kills that target unit before Death from Below resolves.
3. Resolution:
   • When Death from Below attempts to resolve, it checks its target. Because the target is no longer on the board, it is an illegal target (Rule 359.3.e.2).
   • The instruction to "Kill a unit" cannot be executed because the target is illegal (Rule 359.3.e.5).
   • The spell then attempts to check the target's Might to determine if you may play it from your trash. Because the target is no longer on the board, its Might is treated as "null" (Rule 359.3.e.12).
   • Since the condition "if it had 3 [S] or less" cannot be evaluated (it returns null), the "you may play this from your trash" effect does not trigger.

The spell resolves, but because the target is illegal, it fails to perform its primary instruction and cannot evaluate the condition required for the secondary effect.