Undercity Upheaval — ruling 1

You choose how many targets Undercity Upheaval has and how the counters will be distributed as you cast the spell. Each target must receive at least one counter.

Undercity Upheaval — ruling 2

If some of the creatures are illegal targets as Undercity Upheaval tries to resolve, the original distribution of counters still applies and the counters that would have been put on the illegal targets are lost. They won't be put instead on a legal target.

Undercity Upheaval — ruling 3

If all of Undercity Upheaval's targets are illegal at the time the spell tries to resolve, it won't resolve and none of its effects will happen. Creatures you control won't gain vigilance.

Undercity Upheaval : règles, fonctionnement et interactions (MTG)

Undercity Upheaval is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Undergrowth — Distribute X +1/+1 counters among any number of target creatures you control, where X is the number of creature cards in your graveyard as you cast this spell. Creatures you control gain vigilance until end of turn.

How it works : Undercity Upheaval

Original English text — Wizards of the Coast / Scryfall

#1May 12, 2023· Wizards of the Coast
You choose how many targets Undercity Upheaval has and how the counters will be distributed as you cast the spell. Each target must receive at least one counter.
No other card is named in this ruling.
#2May 12, 2023· Wizards of the Coast
If some of the creatures are illegal targets as Undercity Upheaval tries to resolve, the original distribution of counters still applies and the counters that would have been put on the illegal targets are lost. They won't be put instead on a legal target.
No other card is named in this ruling.
#3May 12, 2023· Wizards of the Coast
If all of Undercity Upheaval's targets are illegal at the time the spell tries to resolve, it won't resolve and none of its effects will happen. Creatures you control won't gain vigilance.
No other card is named in this ruling.