The main set-up for this story is that Harry travels back in time to the Marauders era. Bellatrix joins him on his mission to kill Voldemort. The book is divided into two halves (which will become obvious once you have read it). The first half is definitely the stronger half as the set-up, world-building, and characters are all excellent. One thing which this story does quite well, which I find that many others do not, is that the main romance pairing (aka Bellatrix) actually serves a good purpose in the story. She really comes into her own as a character and the dynamic between her and Harry is quite strong. The main secondary characters were all very enjoyable as well, particularly Orion Black and Romulus Malfoy. The second half of the story takes a very different turn from the first half. It is still very good but it definitely has a different feel to it than the first half. It does have many great moments in it though which I found to really help lift up the story. My main critique is with the ending. It feels quite rushed (particularly the final battle) and there are several character storylines which don't really have any resolutions. There is a sequel to this story, though at this point it appears to be abandoned. That isn't a big deal though as this sequel covers a completely new and different plot which isn't very closely connected to this one. Might be worth a read if you really enjoyed the characters in this story and want to see what they are up to years later. I highly recommend reading this story. The characters really are excellently done and they mostly feel strong and fleshed out. While it may not be 100% perfect in some respects, it is one of the few stories which I feel is very unique.
This story actually made the Harry/Belatrix pairing believable (though still changing both characters to a significant extent) which is no small task. Unfortunately the second part was far worse than the first one, dragging on for far too long and making one lose interest in the fate of the characters.
It's been a while since I read this fic, but I may actually return to it after writing this review. From what I remember, Delenda Est strikes me as a fic that was written specifically to pair Harry with Bellatrix, but not be lazy about it like so many others. Effort was put into getting them to a place where it would not be the typical teenager/child paired with an adult like so many other pics. Bella is essentially an OC in this fic, which makes sense, but she is a good OC, which is rare. For that matter, every other character is as well, as we get to see the older generation of the HP-verse in thier school years, but I can't say I remember any to be particularly superfluous, which is useful in a fic centered largely around political manuevouring between the big Slytherin families in the pre-to-early Death Eater days. Harry in this seems to be older and a less-than-canon, due to an overscreen time-skip that I believe predates the last book or two, but acts as intelligent as I believe we were informed he supposedly was in the books, tempered by several years of a running battle against Voldemort pre-fic. I may bump up my star rating on reread, but this is a competent and enjoyable read if any of the above intrigues you.
This one is pretty different from your typical HP fanfic, and deserves recognition just for that. It's not the best thing ever written, but maintains a good balance of realism, and has one of the more interesting methods of defeating Voldemort I've seen in fanfic. If you can overlook the ridiculousness of the pairing, it's worth a read.