Yowch. When used after blockers are declared, this gets you an instant saboteur trigger with your... okay, Skirk Commando is about the best you can get, and then you're just paying for a Lunge. So I guess that's okay.

This does have the drawback that at the moment, you can use it during other players' combat steps. It should probably say "Play ~ only during your combat phase".

made the change which does make it much clearer, also moved it to Uncommon which is a better place for it I think.

Was thinking that maybe it should sacrifice the goblin rather than return to hand. Especially with all the ETB effects out there

Are you intending for the opponent to be able to block the Goblin, or for the caster of this spell to be able to do it after blockers are declared? I think the wording should make it clear either way: either say "Cast ~ only during your declare attackers step", or "Cast ~ only after blockers are declared / only during your declare blockers step" and "tapped, attacking and unblocked".

BTW, Warren Instigator is the scariest target for this I can think of.

I am thinking either when attacker are declared with the card as is

OR

After blockers are declared and sacrifice rather than return to hand.

I've got scarier. How about Siege-Gang Commander, Murderous Redcap, Outrage Shaman. Stingscourger is irritating. Shrieking Mogg could be fun, since you've already attacked.

Or worst of all, Goblin Settler.

Oh, hang on, I didn't spot this was an Instant rather than an every-turn enchantment. Well. I'd still like to get double-duty out of each of those triggers.

OK settled on you sacrificing the goblin at start of next end phase but allowing this to be played at any point during your attack phase - before OR after blockers are declared.