Mastodon - The Hunter. Seeing them Sunday...well, actually mainly going for Red Fang. It's ok, quite melodic but gets a bit samey after about 5 songs. There's only so many noodly guitar riffs and Brann Dailor variations on the same drum fill I can take.
A little more research suggests they may be replacing Priestess on the whole Priestess / Red Fang tour which appears to be hitting Manchester (and therefore presumably elsewhere) on 16th April.