"Until the regroup phase, each minion skirmishing a companion loses fierce and cannot gain fierce."
The grammatical clue here is the comma. It's very subtle, but that's what makes it clear that the "until" clause affects everything after. The difference is
Until Phase X, Y happens and Z happens
vs
Y happens until phase X, and Z happens.
These are both clear. What would be ambiguous would be
Y and Z until phase X.
I have a special deal: "Until tomorrow, if you play a league game, you will receive a foil playset of every expansion."
So if someone does play, he will get the playset, but will have to give it back the following day, correct?
Nope. The difference is that the card text was describing simple effects (being fierce, ability to be fierce) whereas your sentence describes a conditional effects (if you do X, Y happens). Normally "until" just describes how long an effect lasts. In your sentence, you put "until" with the if, not the effect. Thus, you described how long the if-then sequence would apply for, not how long the EFFECT of the if-then sequence would apply for.
The sentence that would get your meaning would be:
"If you play a league game, you will have access to a foil playset of every expansion until tomorrow."
EDIT: Looking at
Harrowdale, it seems unclear what the intent of the card's writers was... I blame the incredibly sloppy way Decipher chose to handle site text.