"So you see, the reckless battle strategy of the Kathorran orcs was effective, but ultimately proved to be a double-edged sword. As did their double-edged swords."

Blade Historian

Attacking creatures you control have double strike.

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If Blade Historian leaves the battlefield after first-strike combat damage has been dealt but before regular combat damage (perhaps because it attacked and was destroyed by first-strike combat damage), attacking creatures you control will lose double strike. A creature without double strike won’t deal regular combat damage if it already dealt first-strike damage that turn.