P'tarix reads:

Action: Corrupt this unit to reveal a Spell or Epic Spell from your hand and put it on top of your deck.

Xirat'p reads:

Action: Corrupt this unit to reveal the top card of your deck. If you reveal a Spell or Epic Spell tactic, you may play it by spending resources equal to its loyalty. Otherwise, discard it.

Beastman Shaman says:

Action: When a Chaos unit you control is corrupted draw a card.

If I corrupt P'tarix to place a card, when do I activate and carry out Beastman Shaman's effect of drawing a card when a unit is corrupted? Do I draw the top card before or after I place a card on top?

Similarly, what if I corrupt Xirat'p, and activate Beastman Shaman's effect? Can I draw a card before or after I reveal the top card?

I think the answer is:

Corrupting the unit is a cost so you can trigger the shaman in response to the corrupting action. Assuming no other other actions enter the chain you will resolve the effects in a last in first out order, i.e. draw a card from the shaman then trigger the unit's effect.

Thanks!