Deceptively strong in singleplayer. One threat for one resource is a pretty good trade when Doomed only has one threat dial to increase. A very useful hero for breaking through the early game, as his 'resource generation' is great in Lore when you often have a glut of cards and nothing to pay for them with. Often nice even when you don't actually use his ability, as simply having him there can free up a resource that you'd otherwise want to save for events you may or may not even need this round (e.g. A Test of Will). The main advantage against him is his stats being worse than expected for his threat cost, so you have to be sure that you'll need his ability. Overall, a very strong contender for any solo Lore deck (especially without Leadership and other resource generation).
Action: Lower the cost of the next card you play from your hand this round by 1. That card gains Doomed 1. (Limit once per round.)
The Voice of Isengard #2. Lore.
Grima is an interesting hero. A lot of people would say he should only be played in solo, though others might argue uncharitably that those who decry multiplayer Grima are just being babies about threat. I mean, here's the thing about threat - most of the time it doesn't matter. It matters to score, but who cares about score, and even then the number of rounds taken to beat a quest factor further into the score than a little extra threat. It matters to engagement costs, but generally that means there'll be one or two points where you want to be careful about it and then you stop caring again because you're past the engagement costs. It matters if you hit 50, but that's a pretty high bar to reach in general. The vast majority of games you don't even come close to 50 threat.
Now, this isn't an absolute. Excessive Doomed can screw up certain strategies, like Hobbits or Secrecy, but no card works with absolutely everything. By and large a little Doomed isn't going to hurt anyone very much, just avoid the cases where it will. The only argument then is if said Doomed is really worth it for the benefit. Now of course the real problem people have with multiplayer Grima I think is not the Doomed itself, but the fact that the Doomed affects all players while only the player controlling Grima benefits from the cost reduction. It's kind of a valid point, but I'd still be inclined to say it shouldn't be so hard to just suck it up and deal with the few extra threat points, though I will concede that it means Grima gets arguably worse as player count rises. Just one more thing to add to the divide between solo/2 player/3-4 player.
Leaving aside the whole "global Doomed vs individual benefit" argument, is Grima's cost reduction worth it? Unquestionably, yes. You get the equivalent of 1 resource for 1 Doomed. Compare this to Legacy of Númenor, which costs Doomed 4 to give each player (usually) 3 resources, suggesting an exchange rate of 1.33:1 for threat to resources, in comparison to which Grima's cost reduction is cheaper. And that's without getting into the particular benefits of this cost reduction. Cost reduction and resource generation obviously are quite similar effects, with a couple of significant distinctions - resource generation has an advantage in that you can trigger it even when you don't have a card to play and then you'll still have the resources in subsequent rounds whereas cost reduction can't be banked for later; on the other hand Grima's cost reduction at least has the significant advantage that it can be applied flexibly to whatever card you want to play, from any sphere, whereas resources when generated have to be put on a particular hero who most often will only have one sphere. That flexibility along with the power of the ability makes Grima easily a top tier hero pick. Another point with regard to cost reduction vs resource generation is that cost reduction can give you a faster start since the resource generation may cost resources to play in the first place - e.g. a deck with one hero and Grima can play Steward of Gondor on round 1 whereas without Grima you'd have to wait an extra round.
Despite Grima's incredible power, I admit I do still wish there was more synergy for Doomed. You don't need it, you can just include a bunch of cards you wouldn't otherwise be able to afford sensibly, but it'd be nice to have one or two more things that specifically worked well with Grima. The things we do have, for completeness - Orthanc Guard technically, though he's not very good; Isengard Messenger is 2-cost 2 if you trigger Grima consistently, potentially 3 if another source of Doomed happens; and of course the Keys of Orthanc net you (or someone else) a resource for triggering Grima in addition to the one saved by the cost reduction, making it a pretty great deal. I'd like one or two more things like that though, that trigger a benefit when the Doomed keyword goes off.
Final notes on Grima: his stats aren't up to much. Most likely he just quests for 2. 2 and 3 could take a hit in a pinch, and of course being in the sphere you can give him A Burning Brand, maybe Protector of Lórien if you want to go that route. It should be pointed out that his threat cost is in fact 1 point higher than the sum of his stats, and I'd say it's debatable to what extent that's merited, but it shouldn't discourage you too much in my opinion. And one little trick you can pull with Grima (which might help your team-mates in multiplayer to complain less) - his abiility is an action which can be triggered in advance of actually playing the card, but the Doomed doesn't happen until the card is played. So for starters triggering the ability in advance allows you to potentially reduce the cost of a Response event where otherwise you couldn't, lacking an action window, and furthermore this means you can trigger Grima speculatively and then the Doomed won't actually happen if you don't actually play the card. So trigger Grima at the start of the quest phase, and then if need be you can play A Test of Will for free (and Doomed 1), but if you don't have to cancel anything the Doomed won't happen. Note that Grima's ability lasts until the end of the round, so if you play a different event later in the round then the cost reduction and Doomed will apply, so remember that (and don't waste it on a 0-cost event, because then your team-mates will get really annoyed). People can be more accepting of Doomed 1 in exchange for A Test of Will, or Hasty Stroke, or Feint than if you just use it to advance your own board-state, and especially so when the event and Doomed may not actually happen if not needed.
Overall though, Grima is an amazing hero and I think more people should make use of him, both in solo and in multiplayer ignoring the futile complaints of the other people around the table (just don't grief them too much with it).