Those damn Daleks have been messing around with portals and wormholes again!
On this occasion, a portal has been opened to Mephisto's realm, and a scout sent to stealthily investigate it. Luckily, old Mephy hadn't noticed, as he was too busy busting out remote threats to a distant galaxy.
Anyway, as you would expect, the scout has established that you don't just attack Mephisto in his realm without some serious prep. So, the Daleks temporarily close the wormhole and use their resources to prep as best they can for a full-on assault of Mephisto's realm. Time isn't really a factor when you're a Dalek, so they take as long as they need and prep to the best of their abilities.
And then the attack is launched; The wormhole is reopened and they pour in.
Can the Daleks take down Mephisto in his own realm?
Edit: Just realised I gave no reason whatsoever as to why the Daleks would want or need to make this attack. So the reason is simply because they're Daleks.