As the countdown to the 2011 festival season continues Sonisphere have added a dynamic dozen to their already talent-stacked bill.

Leading the way are cult rockers The Mars Volta with other standout names including Gus G’s Firewind and retro metal heroes Black Tide – due to release their new record this spring! Straight off the back of completing their sixth (as yet untitled) studio album, The Mars Volta have confirmed their return to the UK for an exclusive performance at Knebworth. The El Paso Grammy award winners will be bringing their unique blend of Latin-American infused psychedelic prog rock to headline the Saturn stage.

Over the last decade, The Mars Volta have earned huge critical acclaim and kudos from both musicians and media alike, with musical masterpieces like Inertiatic Esp, Televators, The Widow, Wax Simulacra, Teflon and Since We’ve Been Wrong dropping jaws the world over.

Guitarist Omar Rodríguez-López said: “We’re stoked to come and play. Everything I’ve heard about the festival has been positive, I’m sure it’ll be great. Playing live for me is the best part of being in a band, I can’t wait to get back out there. I’m excited to play the new songs.”

Another band making their Sonisphere debut is Sweden’s mighty Opeth. Having formed more than 20 years ago, Opeth have pushed the boundaries of a plethora of genres by seamlessly incorporating elements of prog, folk, experimental jazz and classic rock with a style firmly rooted in Scandinavian death metal, creating a legacy of sound that consistently evolves and excites. Frontman Mikael Åkerfeldt said:

“We’re chuffed about the invite to play Sonisphere in the UK. Not sure who else is on the bill, but I guess that info is just a click away.

“Regardless, we’re coming to play to you, first and foremost and anything else wouldn’t matter at the end of the day. Always loved playing to the UK crowds, and I’m sure this gig will be no exception. See you there!”

Killing Joke are no strangers to Knebworth, having performed at the inaugural Sonisphere in 2009. This time, the original line-up of Jaz Coleman, Geordie, Youth and Paul Ferguson will take to the Bohemia stage after the grand finale of The Big 4 on the Friday to show why, after 30 years, they’re still one of the most important bands around.

Hugely influential to Friday night headliners Metallica – who covered The Wait on their Garage Days EP – as well as a host of iconic acts such as Nirvana, Ministry, Prong, Tool, Nine Inch Nails and many more too numerous to mention – Killing Joke will leave you in no doubt as to why, with songs from their illustrious history and 2010’s incredible return to form Absolute Dissent.

Returning to the UK after a main support slot on the Kerrang! Relentless Tour, Massachusetts easycore quintet Four Year Strong will be joining Sonisphere’s Sunday line-up on the Bohemia stage. With an eclectic sound borne out of a desire to create music that the band themselves would want to hear, Four Year Strong skillfully blend melodic hardcore and screamo vocals with power pop synths and numerous nods to 90s pop-culture.

And it sounds like they can’t wait to get to Knebworth with guitarist and vocalist Dan declaring: “I’m so stoked to play Sonisphere, great lineup of bands to be a part of. I’m excited to play for an amazing fest in an amazing country.”

Added to Friday’s line-up are Michigan’s melodic death metallers The Black Dahlia Murder. Pounding Swedish-style blackened thrash with American death metal muscle, the band is set to lay waste to the Bohemia stage with the relentless, merciless brutality that has put them at the forefront of the underground metal scene. Frontman Trevor said: “We in The Black Dahlia Murder camp are completely honored to accept an invite to the amazing Sonisphere this year.  Seeing The Big Four live has been a dream of mine… playing alongside them…well, kill me now!”

Despite being amongst the youngest artists to feature at this year’s Sonisphere, Miami’s Black Tide have already secured their position as worthy contenders of the metal scene. With their 2008 debut album garnering instant acclaim, leading to them being named Best International Newcomer at the Kerrang! Awards and achieving a top eight single in the US Billboard Top 100, the band continue to ride high.

Support slots on Iron Maiden’s up-coming US dates have already been confirmed and a sophomore album, featuring guest vocals from Matt Tuck of Bullet For My Valentine, is set for release this spring.

Controversial Swedish black metallers Watain are looking forward to bringing a sinister edge to this year’s Sonisphere. Indoors or out, rain or shine, the fearsome five-piece will ensure their set is enshrouded in an inescapable darkness, as they deliver a grim, blood splattered set. Mainman Erik Danielsson enthused: “Playing alongside Metallica – one of the greatest musical influences of Watain – has always been a dream of ours, so this feels like a most pleasing reward for walking this long hard road all these years.

“Watain will be a dark and fierce addition to this already mind-blowing lineup of metallic overkill, and we look forward to bring our blood, fire and death to the UK visitors…”

With a CV boasting contributions to Dream Evil, Arch Enemy, Mystic Prophecy, Kamelot and Nightrage – not to mention his role as current axeman to Ozzy Osbourne – Greek Guitar Guru Gus G is one of the most high-profile young guitarists in the world today.

But his own band, Firewind, has been the one constant in his hugely successful career. After six albums and 10 years, the Greek power-metallers are set for their first appearance at Knebworth this year, and promise big things for the Sonisphere crowd.

“We’re very excited for our debut performance at Sonisphere festival,” said the band. “We’re truly honoured to be a part of Sonisphere UK and to share the stage with the Big Four and so many other great bands!! Knebworth get ready, we’ll set your world on fire!!”

Returning to Sonisphere this year are London metalcore outfit Rise To Remain. Having won the Metal Hammer Golden God Award for Best New Band and the Kerrang! Award for Best British Newcomer in 2010, Rise To Remain have gone from strength to strength, touring extensively with, amongst others, Iron Maiden, Korn and Hatebreed, and are set to release their debut album later this year.

Frontman Austin Dickinson said: “Sonisphere this year looks set to destroy all in the immediate vicinity, and we can’t wait to raise hell with them! Pints and horns ready my friends, this is going to go off!”

Bringing tales of rum, wenches and high-seas rolling to the grounds of Knebworth are Scottish pirate-metallers Alestorm.

“Avast ye fannyfaces,” said the band. “We’re dead chuffed to finally be invited to Sonisphere. We’re gonna get drunk pirate style and party like it’s 1692! We might play a few songs too, if we’re not too hungover. See you all in the pit…”

Having last toured the UK in late 2010 with Sabaton, Alestorm will follow up the May release of their third studio album with their first Sonisphere outing.

Also appearing are Cambridge Rockers The Treatment, who have just released their debut album via Powerage Records, and Six Hour Sundown, whose frontwoman Lauren Harris added: “We’re thrilled to be playing Sonisphere this year. It’s a great festival and the line up is amazing! What better place to announce the arrival of Six Hour Sundown. We’re playing the Red Bull stage and we can’t wait to see you there!”