"Blindside" Custom Mechanic
Alright, let's try a mechanic like this:
Blindside: This creature can attack during your opponent's combat phase.
Your job is to create cards using this mechanic, in any way you see fit. As with my previous little experiment, there are no prizes aside from fairly liberal favorites for cards that I enjoy. Creativity, flavor, and an awesome factor are encouraged.
Here's an example on a card (I made it quite some time ago):
Blindside: This creature can attack during your opponent's combat phase.
Your job is to create cards using this mechanic, in any way you see fit. As with my previous little experiment, there are no prizes aside from fairly liberal favorites for cards that I enjoy. Creativity, flavor, and an awesome factor are encouraged.
Here's an example on a card (I made it quite some time ago):

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Comments
https://mtgcardsmith.com/view/haldrans-crusader
Ooo very nice. I like that stipulation that could be very powerful depending on circumstances.
It can block if it doesn't attack. If you're attacking, you're declaring it as an attacker, and so it can not block. Standard combat rules apply, except that now you can attack as well as your opponent.
My assumption is generally no, unless the creature has vigilance, due to the fact that declaring attackers comes before declaring blockers.
@syntheticreign , what makes you think that just because you declared it as an attacker that you can't also declare it as a blocker, assuming it's still untapped? When you break open the combat rules, these are the sort of things that come out.
Oh that's right! I totally forgot haha. Although I'd still say that by the nature of attacking and blocking, you forego one for the other.