MArcomage

I think this is a bit too situational. Most times you would discard at most one card at a time, and the stock gain would be too low to contemplate for the high cost of this card. The only good thing about it is the Legend keyword, but still if I would decide to play with Legends, this would not be a card of my choice for my deck.

Maybe the idea behind it is to combo it with cards like Auxilia, which would be quite devastating.
Just to counter Banish and Charge it makes no sense, but when building that kind of combos...

The card's Note confirms that it's only meant to work with cards it itself is discarding, which is how I would have assumed it to be from the card's text.

This one is a tough call. When used properly, the effect can be huge. The question is whether anyone would ever really get to see it happen.

I'd suggest lowering the cost and capping the effect, though I don't know what values to suggest without studying all existing Charge and Durable cards to get an idea for the average amounts one can expect from this.

Since Skirmisher and Banish only discard 1 each, in combination with the only-works-for-that-cards-thing, it's really too expensive. Else it's potentially awesome.

Perhaps allow it to discard randomly 2 cards from opponent's hand in addition to those discarded by Banish & Skirmisher?

It's not implementable in current form. You can't reference results of keyword effect with card effect, since card effect takes place before keyword effect.

Then remove the keywords and have the effect take place in the card's description?...

Given that the old form is unimplementable (card effect may not reference keyword effect), I've revised the text substantially to save the flavor as much as I can.

Old text:
25b 25g 25r
Legend. Banish. Skirmisher. Brigand. Swift.
Gain stock equal to cost of all discarded cards.
Opponent loses stock equal to cost of all discarded cards.

New text:
15b 15g 15r
Legend. Brigand. Swift.
Discard all opponent's common cards.
Stock: +N
Enemy stock: -N
N= total cost of all discarded cards

This new form should definitely be implementable, and it keeps the flavor of Hermes being the god of thieves of legendary speed. The new version allows the opponent to redraw after they lose their commons, but this drawback seems to be more than made up for in the card's effect.