3 games played so far with Innsmouth Horror and I can certainly confirm the opinions of those who say it adds a significant increase in difficulty!

Last night we ended up in battle with Rhan-Tegoth and were a little confused with his attack:

Attack: Rhan-Tegoth drains the blood from a victim. The investigator with the highest current Stamina (first player breaks ties) rolls dice equal to his Stamina and loses 1 Stamina for each failed die. For each Stamina Rhan-Tegoth drains, the investigators must inflict one extra success on him in order to defeat him.

So there were four investigators, requiring 4 successes per doom token. The AO attacks draining 3 stamina from an investigator. Does this mean that I need to get 7 successes for the next removal of a doom token (and 4 successes for subsequent doom tokens) or do I need 7 successes for the rest of the game?

I may have follow up questions based on the response so be warned

You need 3 additional successes, on top of however many are left, to kill him. Basically, add the stamina removed from the investigator and add it to the AOs sheet. Next time you get a success, remove one of those stamina tokens. Once all 3 are gone, continue with however you keep track of how many hits are left before a doom token is removed or how much HP he has left.

So, in summary, he needs 1 more one-time success each time a stamina is "drained."

This is why in Tib's stats report, it is the hardest AO to beat; it's nearly impossible to take this guy down.

Just one point I have to make: treat the Stamina loss by Rhan-Tegoth as unable to be prevented, or else McGlen would be invincible.

It appears that we actually got it right and fought correctly!

This was my first time fighting Rhan-Tegoth and luck was with us. We had Agnes Baker with a healing spell

Agnes Baker: Blood is Power - Any Phase: When paying a spell's Sanity cost, Agnes may spend Stamina for part or all of the Sanity cost on a 1-for-1 basis.

Heal: Casting Modifier: +1 Sanity Cost: 1

Upkeep Phase: You may cast and exhaust. You or another investigator in your area gains Stamina equal to the successes you rolled on your Spell check. This Stamina cannot be split between multiple investigators.

I was able to cast the healing spell and never run out of sanity because I could spend stamina instead - and then heal myself.

Without this ability we simply would not have managed it.

Thanks for the clarification