Let's go through this one at a time, shall we? Elseworlds is basically a way of authors saying "what if?" and re-imagining famous heroes in different ways. I've actually read the entirety of the Batman Vampire story and quite enjoyed it, but this anthology offers up a mixed bag to say the least.
Holy Terror- Imagine if the US was a Theocracy, with fundamentalist Christianity reigning supreme (depending on who you ask, it already does!). Batman is a priest in this one, but also seeks vengeance on the church system that had his parents executed. There's some interesting stuff at play here, with him having a crisis of faith and trying to decide how to maintain his faith while bringing down the system that uses that same belief system to oppress people. It sadly doesn't live up to its potential. Batman learns that a member of the state is experimenting with genetics to create what essentially amounts to the Justice League, but there is no resolution to this story. It ends abruptly, with little sense of closure.
The Blue, the Grey, and the Bat- Bruce is a member of the Union army who works closely with Abe Lincoln to fight the rebellious south, and he wears a bat costume while he does it. To be honest, this was just so goddamn absurd and stupid that I kind of loved it. It's not very good, but it's extremely entertaining, even though a lot of names get thrown around and it can be hard to follow at points. But for God's Sake, there's a group of freed slaves that now fight the south called the Dark Knights, and that is so horrendously tasteless that you have to read it.
Robin 3000 1 and 2- Set in a dystopian future where Batman and Robin are descendants of the originals, except this time they're freedom fighters who plots against the Skulp, a race of aliens that have enslaved humanity by making them believe that they're being treated well. This one was bizarre, and while I understand that the point of these stories is to posit some what ifs, this barely felt like a Batman story at all and instead seems like an idea the writer had for a space opera that they slapped the Batman name onto to get made and like Holy Terror, this one does not technically have a resolution, as the immediate villain is defeated, but humanity isn't free or anything by the end. Probably the weakest entry in this anthology.
Batman/ Dark Joker: The Wild- This is a surprisingly bloody dark fantasy where Batman is a creature created by two magic users to combat the Dark Joker, a homicidal and maniacal wizard that seeks to wipe out the world. This one, despite somewhat generic high fantasy art, was pretty enjoyable for me, as it was really violent and had some interesting themes and surprising body horror.
Batman/ Houdini: The Devil's Workshop- Set during the heyday of Houdini, this odd tale sees Batman team up with the renowned magician to investigate the disappearances of children all around Gotham. Using a gorgeous art style that really helps sell an oppressive atmosphere, this strange but effective tale is once again surprisingly gruesome, with genuine, hard hitting brutality that doesn't pull punches. It could be seen as a companion piece to Gotham by Gaslight due to its period setting, and it's probably the most effective tale in the collection.
Castle of the Bat- Essentially a retelling of Frankenstein where Bruce Wayne is the doctor and the monster is actually the Batman. It's a weird mash up, to be sure, and at times it's eye rolling, but the art helps sell the kind of stupid concept, making it work far better than it should be, and this is coming from someone who couldn't get through Frankenstein.
In Darkest Knight- Essentially, Batman if he was apart of the Green Lantern Corps. From what little I know of that DC series, I don't particularly care for the lore or characters, and this basically just turns Batman into a space travelling Superman, with all that implies. The one cool thing is that Sinestro, (a famous Green Lantern villain IIRC) fuses himself with Joe Chill, leading to what could have been an interesting tale, but it's mostly just shenanigans until the end, which, as a common theme with the worst bits of this collection, is a cliffhanger that doesn't resolve anything.
Dark Allegiances- Set right near the end of the 1930's, this is a noir style thriller where Batman uses the money of corrupt and outright racist senators who belong to the KKK to fund his nightly hobbies. Eventually he gets entangled with a conspiracy to destabilize US relations with other nations during WWII. This one was convoluted and tough to follow at times, but was entertaining enough. Plus, the opening where he invades a KKK rally is absolutely great.