I originally thought that this card would be broken, but after putting into practice for a campaign, I think that it is fairly costed. It's a one-of that is hard to find in your deck, costs 4 resources, requires you to do a little work in the first scenario you find it, and it costs 4 XP. Let's talk about that last point for a second -- 4 XP is a lot and usually gets you cards that progress the scenario. In Seeker, it certainly buys you strong cards. You can get a fast free draw 3, an ally that will provide some soak and double the effects of some Tomes, a fast event every turn, and six (or more) extra actions.

What does Gateway to Acheron do? You have to spend an action to move anyone to 4 locations -- notably ones that have already been revealed. On the biggest of maps, if you really want to move to the far corners more than once, this will save you quite a few actions by itself. In practice, most of the time you're going to move two or three locations away, and you might use it on a friend who doesn't have any actions left. This is very useful! But Arkham Horror has been pushing movement cards for the last few cycles. We have Scout Ahead that moves you up to 3 times. Seeker already has Join the Caravan to perform this effect once without spending any actions. Even Astral Travel will get you across a map at Level 0 and has been in the card pool for ages. And usually scenarios with big maps have scenario effects that let you move a little more quickly.

Now, the other part of this card in particular is to take a free investigate action at the cost of removing the leyline. In practice, given the pressures of the scenarios, I used this ability in conjunction with the movement almost every time, and always on myself (I was the seeker after all!). You still have to take the test, and it could be using any asset or event to do so. But again, you still have to spend an action to trigger the ability in the first place, so in practice this felt like I was saving 2-3 actions every time I used it. Very good, and probably a bit better than the other 4 XP cards, but not overly so. You're more likely to get the obvious uses out of Eon Chart, whereas this one will sit on the table (or your hand) until it feels like you'll get the most value out of it. Still, this card is very good; great, even, but plays fair. I also found it fun to decide when was the right moment to use it, unlike Eon Chart which you pretty much just use every turn you have a secret on it.

Finally, a comment on "breaking" certain scenarios -- yes, a few certain older scenarios expect you to trek across a map, one location at a time. But I don't think Arkham Horror cares if some scenarios are easier than others if you tech for them or get lucky. Luke Robinson's entire schtick is that he can warp anywhere starting from the beginning of the campaign. There are plenty of other cards, even at level 0, that allow you to avoid or trivialize entire acts -- heck, "I'm outta here!" is designed for this. The balance is that some scenarios make Archive of Conduits fairly useless. If you have a small map, or no reason to go to revealed locations you've already visited, this is a 4 XP card sitting in your hand. At least in that case it has excellent icons to commit.