Counterspell Contest

You attempt to interrupt a creature in the process of casting a spell other than counterspell. (You know the level of the spell being cast, but not the specific spell.)

When you cast Counterspell Contest, any other casters who can cast it may do so to join in. The original caster is always included and doesn't need to cast anything else to participate. 

Each creature participating in the Counterspell Contest declares what level spell slot they're using (minimum of 3rd level slot). The original casting creature may uplevel the slot they're using for their original spell, and other participating casters can choose to use a higher level slot as well.  All participants can raise until they're not willing to raise further. No one can lower their bid or withdraw after joining in. Everyone who participates will lose the slot that they end up using in the contest.

If the spell slot used for the Counterspell Contest is equal to or higher than the slot the original caster is using, you have advantage on the contested check. If the spell slot is 3 or more lower than the original caster's slot, you have disadvantage on the contested check.

All participants make a contested Intelligence (Arcana) check.

• The caster with the highest result may choose to cause the original caster's spell to fail and have no effect.
• If there is a tie for highest result, and the tied casters don't agree whether to cause the spell to fail, the original caster's action is wasted, but they do not expend their spell slot.
• If a single winner's result is 10 higher than any other result, the winner can choose to change the target of the spell as if they were the one to originally cast it.

Counterspell Contest never triggers Counterspell.