Wikipedia sums up the plot (here) as...set on the Hive World of Necromunda, The Redeemer chronicles the battles of a small band of Redemptionists, fanatical and homicidal cultists of the Emperor, led by their supreme leader and arch-maniac, Klovis the Redeemer, as they fight to stop the Caller, a deadly and extremely powerful Ratskin Shaman, who is leading a horde of Ratskins, Scalies, Plague zombies and other Underhive Scum in a revolution against the nobles of the Spire.
Inspired to read more about a chainsaw wielding maniac with a burning brazier on his head (whose catchphrase isn't quite 'be pure, be vigilant, behave' instead it's a deviant-taunting 'scourge and purge')? Well this is where to start...
In the Summer of 1999 Warhammer monthly went bi-weekly so that 3 special issues, featuring just material related to The Redeemer, could be published...
Issue 16, June 1999
Issue 20, August 19999
Issue 22, August 1999
And this is what you got for your money
Issue 16 = a one-off introductory story
Issue 18 = chapters 1-4 of The Redeemer & a Wayne Reynolds interview
Issue 20 = chapters 5-8 & a Debbie Gallagher interview
Issue 22 = chapters 9-12 & a Pat Mills interview & a Wayne Reynolds interview
All of the above is black & white art
The (black and white) work was then collected in this paperback volume in 2000, yours for £7.50, ISBN = 1-84154-120-6
Then in 2002 the main series (so ignoring the material in Warhammer monthly #16) was coloured and published in its own stand-alone mini-series (only one other Warhammer strip got this special treatment but that's a story for another day). So now you have...
June 2002, cover recycled from Warhammer #22; chapters 1-3
July 2002, cover recycled from Warhammer #18; chapters 4-6
August 2002, cover recycled from Warhammer #20; chapters 7-9
September 2002, cover recycled from Warhammer #16; chapters 9-12
This coloured material (plus a previously unpublished strip) was then itself collected, in 2002, in this fine volume, yours for £12.95, ISBN 1-84154-274-1
Note that in story-telling terms The Redeemer story arc is made up of 3 'books' - chapters 1-4 are Book 1, chapters 5-8 are Book 2 and chapters 9-12 are Book 3.