Let's be short. Thengel is AMAZING! No one can deny that.
But, for sake of this being a good review, I'll elaborate.
First, let's look at his statline: 11 , 3 , 1 , 3 , 4 . Kind of lopsided, and reminds me of Elrond, who is 13 , 3 , 2 , 3 , 4 , which comparing their stats, the only thing that Elrond beats Thengel in is , which is rarely used. And Thengel has 2 less than Elrond. Comparing further,Elrond can get Light of Valinor and still utilize his 3 . Similarly, Thengel can get Snowmane and use his 3 . But when it comes to cards to boost the stats (Vilya does not boost stats, if you were wondering), Thengel beats Elrond because of the glory that is Golden Shield. It gives Thengel double his starting , and that isn't using other or boosting cards (The Favor of the Lady, Silver Circlet, Dúnedain Quest, Song of Hope, ect.). Thengel trumps Elrond with stats and stat-boosting cards.
Next, let's go with effects. So an ally is discarded from play, (normally Rohan), and you search the top five cards of your deck for another ally that share at least one trait with the ally that left play. If you find one, you play it for free. You can't use it until the end of the round, unless you ready it with something like Ever Vigilant, but it is still free. The only cost is to discard an ally from play, which, in a Rohan deck, which Thengel will mainly be used in, discarding gives you something good anyway. Again I will be comparing Thengel to Elrond. Elrond's Ring effect requires you to exhaust Elrond to play the top card of your deck for free, but at the price of a hero exhaust, which doesn't benefit anything. And Elrond's Ring effect is the top card of your deck, which a lot of times is blind, granted there are cards like Imladris Stargazer, Gildor and Gandalf's Search to look at your deck. But I find playing Elrond/Vilya decks those cards take a lot of setup and I can't always get them out until late game. But in Rohan decks I can discard allies pretty much infinitely, and Gamling makes it even better.
Overall Thengel is almost too good, and him having great art from Unknown Artist is just great.