Colonel Autumn

Lifelink
Exploit (When this creature enters, you may sacrifice a creature.)
Other legendary creatures you control have exploit.
Whenever a creature you control exploits a creature, put a +1/+1 counter on each creature you control.

Fallout

Lifelink
Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
Other legendary creatures you control have exploit.
Whenever a creature you control exploits a creature, put a +1/+1 counter on each creature you control.

Lifelink, exploit
Other legendary creatures you control have exploit.
Whenever a creature you control exploits a creature, put a +1/+1 counter on each creature you control.

A creature with exploit “exploits a creature” when the controller of an exploit ability sacrifices a creature as that ability resolves.

If Colonel Autumn isn’t on the battlefield as an exploit ability of a creature you control resolves, Colonel Autumn’s last ability won’t trigger. You won’t get +1/+1 counters, even if you sacrifice a creature.

You can sacrifice the creature with exploit if it’s still on the battlefield. If it has an ability that triggers when it exploits a creature (or, in this case, when any creature you control exploits a creature), that ability will trigger.

You can’t sacrifice more than one creature to any one exploit ability.

You choose whether to sacrifice a creature and which creature to sacrifice as an exploit ability resolves.