How do you summon a boss as a spectre?

I say a streamer using a boss enemy (Kadaku from act 1) as a riased spectere and was wondering how I could make something related? Is it even possible?

Not sure you can. Are you sure it was Kaduku? One of the corpse item spectres looks a lot like Kaduku. Spirit of fortune. Might of been that one? It's a pretty good one. Looks a bit like a lightning Kaduku.

Perfect Spirit of Fortune
Casts Shock Nova
Casts Spark
Casts Lightning Warp
Casts Wrath
Lightning Damage of nearby allies is Lucky

Possibly,

How would I summon that though? I haven't been able to find it anywhere

It's an item.

https://www.poewiki.net/wiki/Corpse_item

They are tradeable so you can buy them on trade (Think trade is down for maintenance currently). Just make sure you get the one you want, I'm not certain that is the one you are talking about, there could be other spectres that look like a totem. Fairly sure you can't raise unique enemies as spectres though.

They are one use only as well, it does not store in your spectre library, so if it dies you will need to get another one.

Thanks a bunch