So do I get this right? This thing can never be evaded (because it always untaps) and it deals that damage to each investigator because Massive = engaged with all !?

1) It can be evaded and exhausted DURING an investigator's turn. It just won't stay that way at the end of said turn. You can still spend actions to evade it if you have cards that key off evading enemies (like Pickpocketing and Sneak Attack). But, once your turn is over, it will ready and re-engage.

2) The massive keyword indeed means it will engage every enemy at its location and make an attack against each one during the enemy attack step. You'd better be packing a lot of Dodges or have some other plan to deal with this fellow (I recommend a Shotgun, a Police Badge, some Extra Ammunition, and lots of combat boosters).

I have not killed him yet.  He seems sufficiently difficult for an AO in my opinion.  I do suspect Shotgun features prominently in any tactic to take this guy out in the core only environment.  Six evade is horrid so I can't see Backstab being an easy route to take against him.

Six evade is horrid so I can't see Backstab being an easy route to take against him.

What do you mean by this? Backstab only uses your agility instead of your combat, the fight value of the foe is still used.

Are you thinking of Sneak Attack?

No I definitely meant backstab.  Backstab deals +2 damage on the attack, which is great for beating down big enemies.  Also usually an enemy's evade is lower than their attack, and for anyone who would take Backstab their agility is higher than their combat stat.  Therefore it normally is a very powerful combat card.  Though against Umordhoth it's just too difficult usually to beat a 6.  Especially considering the chaos bag configuration in scenario 3.

Since you brought it up Sneak Attack is also hard to use for the same reason and that costly evade action is less valuable considering Umordhath's Massive keyword and Forced reaction.  Also he's elite so a lot of evade tricks just don't work on him.

Umordhoth is appropriately challenging in the core set it seems for an AO.  You really feel like you should deal with him in some other way if you can help it.

http://arkhamdb.com/card/01051

- Backstab makes you test your Agility skill instead of your Combat skill against the enemy's Fight value (not its Evade value).

The [winged shoe] icon refers to investigator's Agility, not to an enemy's Evade, as per the last page of the Learn to Play guide.

It's frustrating to read your response Mplain because I know all that.  Not sure why I said that about the 6.  I guess I misremembered the card text. 

Also usually an enemy's evade is lower than their attack

What was that about then? oO

Me misremembering the card text on Backstab like I said and you turning the dagger   Specifically I thought I remembered it initiating an Evade action, but it's clearly a Combat action.

What happens if he gets mindwiped? Does he lose the bonus life? What happens if he doesn't have "massive" for one phase? Hm.

Nice try, but you cannot mindwipe it because it's Elite