"Each night, the sun unravels and blows away. Each day, the spiders set a new one in the sky."
—Sunseeder myth

Ember Weaver

Reach (This creature can block creatures with flying.)
As long as you control a red permanent, Ember Weaver gets +1/+0 and has first strike.

The List

Masters 25

Reach
As long as you control a red permanent, Ember Weaver gets +1/+0 and has first strike.

Conflux

Reach (This can block creatures with flying.)
As long as you control a red permanent, Ember Weaver gets +1/+0 and has first strike.

As long as at least one permanent you control is the specified color, the ability will "work" and grant this creature the bonus. Otherwise, it won't have the bonus.

Whether Ember Weaver has first strike matters only at the point that the normal combat damage step would begin, and only if there hasn't already been a first-strike combat damage step that turn. If any of your creatures has first strike at that point, a first-strike combat damage step is created. -- If Ember Weaver has first strike at that point, it will assign and deal combat damage during that step (even if it loses first strike before that damage is dealt) and it won't deal combat damage during the normal combat damage step. -- If Ember Weaver doesn't have first strike at that point, it will assign and deal its combat damage during the normal combat damage step (even if it gains first strike before then).