In the late 90s, it was hard to catch up on wrestling storylines that had gone by. Obviously, there was no streaming, and releasing weekly wrestling TV shows on VHS would have been absurdly expensive and impractical. There were, occasionally, character-specific Tapes that would get released for the most popular wrestlers, containing highlights from the past year or two, and if you're lucky, maybe even a few complete matches. I have two of these old tapes (well, technically, I have the DVD rereleases they got when the format was new and they were just trying to get as much stuff out there for people to buy as possible), and while the other one, Three Faces of Foley, is fairly conventional in its presentation, showing promos and matches between out-of-character segments where Mick Foley bestows fatherly advice upon young Matt and Jeff Hardy, The Phenom takes a totally different approach.
There's no new video footage on this release, for example, nor are there any interviews with The Undertaker, in- or out-of-character. What we get instead are summaries of his feuds from 1996 up to the end of 1998, with Mankind, Kane, Shawn Michaels, and so on. We get to see the most dramatic parts of matches, as well as various promos and vignettes and other storyline stuff. The masterstroke, though, is in the presentation: dramatic orchestral music has been added to almost everything, and each new segment is introduced with dramatic narration from Classy Fred Blassie.
This is all incredibly effective, and makes The Undertaker, Kane, Mankind and Paul Bearer into the straight to video 90s horror icons they were always meant to be. And this DVD ends just before the formation of the Ministry of Darkness, so you get all the insane familial drama, the magically-summoned flames and bolts of lightning, and all that cool supernatural stuff that's sorely missing from current mainstream wrestling (I know there's Bray Wyatt, and he's both a talented guy and an awesome character, but the way they're booking him is atrocious).
It's been out of print for well over a decade, and it doesn't look like WWE have any interest in uploading any of their (or ECW's, for that matter) home video-only stuff to the network, but if you can track a copy down at a reasonable price, The Phenom is an excellent way to spend an hour.