This card continues the tradition of Against the Shadow magic cards being much, much nicer than the original MELE magic cards. While it can't beat The Tormented Earth for all-around coolness, within specific decks it can be highly effective.
The first and most obvious limitation on this card is that it requires a sorcerer; this means that it can be used very easily with most Ringwraiths, but only a few others. See my The Tormented Earth review (COTD #599) for details.
Riven Gate has two closely related effects which, in most cases, will both be useful. But one of the things that makes the card cool is that, in a pinch, you can always use the card for one or the other of the effects if you already have the other one covered. First effect: wiping out the auto-attack on a border-hold. This is something of a small bonus, since the chances are pretty high that the auto-attack won't be too scary: each character facing 1@7 detainment seems pretty average. It could conceivably be worse if you're using this card with an overt company (not too likely) or if the auto-attack is being boosted with Rank Upon Rank or Arouse/Incite Defenders, in which case being able to cancel it could very well be the most important part of this card. Even the everyone@7 can be tricky - let's say, for example, that you have a Horseman who's going to do the recruiting that turn. Untapped he's only at 0 prowess, and while he has better than even odds to make it past a 7 detainment attack, his odds aren't much better than even. It's a bit too much of a risk - a risk you won't have to worry about if you're using Riven Gate.
More importantly, this little spell yields a +2 bonus to an influence check against a faction, and minions need all the help they can get in that department. One nice thing is that the sorcerer doesn't have to be the influencer - it just has to be someone else in the company. Consider the situation with a sorcery-using Ringwraith trying to recruit Woodmen or Men of Dorwinion or Hillmen (all with a 10 target number): in Black Rider mode, Dwar has +5 DI, which gives him a good chance to recruit the faction but leaves an uncomfortable margin for error. Riven Gate's +2 brings things into the realm of Pretty Bloody Likely. Similarly, a Horseman with a Blazon of the Eye has a good but not great chance to recruit one of the above factions, a chance that's pretty much clinched with Riven Gate. And, since the bonus isn't skill-specific, Riven Gate can be helpful in a pinch, when, say, Hador himself is the only one left to make the influence attempt. At least with RG he'll have a ghost of a chance.
The Grimburgoth is big and cool and can use sorcery, so it may be tempting to have him roam alone and knock down gates all through Middle-Earth - or maybe he finds himself all alone because all his companies were killed by Elves. In any case, using RG alone can be tricky - the corruption check all by itself isn't much of a risk to one's LIFE (you're only eliminated on a 2), but there's a much greater chance of the sorcerer tapping, in which case there's no one left untapped to reap the benefits of RG's effects.
Another caution about the corruption (and this goes for most sorcery spells): it's not scary in and of itself, but it does mean that you have to keep your sorcerer unencumbered with other distractions, or you could be losing him in a hurry.
One final limitation: the only places with big enough auto-attacks and hard enough faction checks to make RG worth its weight tend to be in the north. Sure, there are plenty of southern border-holds for minions, but do you really need to cancel everyone@5 detainment automatic attacks, or need an extra +2 to score that >8 faction? Usually not.
Original card review taken from : http://fan.theonering.net/morgulrats/
With the authorization of the webmaster.
The reviewing team consisted of Gwaihir (Chris Farrell), Gimli (Nathan Bruinooge), Ohtar (Charles E. Bouldin, Esq.), Radagast (James Kight), Joshua B. Grace (Beorn), Martijn Steultjens (Fram Frumgarson), Jason Klank (Saruman) and Jeffery Dobberpuhl (Wormtongue)