What is considered a target for the purpose of the Shadowfell shard?  E.g. If I cast Tasha's Caustic Brew with Transmuted spell, is any creature within the spell's line of effect considered to be a "target" of the spell, or is there no targeted creature since the spell targets a point on the map and not a creature?  I have a feeling I know the answer, but all of the threads I've found thus far are asking about the order of operations for the shard and not if it can be used on a creature within an area effect.  Thanks in advance!  https://www.dndbeyond.com/magic-items/2408242-shadowfell-shard

The target is a creature affected by your spell. If your spell affects multiple creature then you choose one.

Yepper, you just choose which one gets to roll at disadvantage, the others roll as normal. It grants the 3-point heightened spell metamagic in addition to whatever metamagic you used. It is a massive savings on a powerful metamagic ability when you are burning through points. I personally use it coupled to subtle spell to help my save-or-suck spells stick better.