Unity is supposed to be instant speed only, to use unity you to cast this card in response to another creature spell you've cast. I do not know if that is clear in the wording.

You might want to give it flash if you're meant to cast it in response to something. I like this unity idea. I think you're wording is mostly right but there's usually multiple ways of wording new ideas like this. As it is now, its not completely obvious that you can cast it in response to another creature spell.

Flash

Unity 1 (This spell costs less to cast if you cast it in response to another creature spell you own.)

Also I think this is strictly better than Benalish Marshal whom is already very strong. It could be interesting if you made it so that creatures entering the battlefield ,while you control this card, enter with a +1/+1 counter.

THanks! I'll integrate flash into the reminder text as that was the intention.

and the card's design is in the same field as Benalish Marshal yet, but I don't know about strictly better. Benalish Marshal can probably enter a turn sooner barring dropping Memnite the same turn as playing this card, and that's in the scenario of annoying, but not a high threat considering the scenarios where that play could occur and what the play would be made against.

Edit: I missed the "other" portion of Benalish Marshal.

The issue with the current reminder text is that you cannot have a hidden card (e. g. in your hand) utilize a triggered ability.

But you can have a static ability in its place:

> "(As long as you control a creature spell, you may cast this spell for less.)"

Since you cannot cast a creature spell normally with a spell on the stack this wording also can be considered to implicitly give flash.

If the triggered ability worked no flash would be necessary either since being cast as part of the effect of an ability always overwrites timing restrictions, if triggered abilities worked at all.

Maybe you can actually make it more like splice instead i. e. pay an additional cost and add this creature spell to another creature spell.

If you went for the static effect, you could make it;

(As long as you cast a creature spell this turn, you may cast this spell for less)