Darragh Wrathe is an incredibly powerful addition to almost any list at, or above, 35 points... why... because he's a force multiplier with near limitless potential. So what can this man do for us, or as the question refers to... what can he do for a caster.
- He's on a large base, and incredibly hard to kill. This means that if Eiryss got to your caster, or clouds/stealth won't work, you still have a place to hide. This might not seem like much, but it's a very nice backup plan when something hits the fan. Darragh can also shield other important models.
- When combined with Tartarus, he increases the Bane Thrall threat range to 11.5", which is the critical inch where they suddenly have a longer threat range than the classic speed six model with reach. This is so incredibly powerful and has won me several games.
- When you mess up positioning, Darragh Wrathe saves the day. Suddenly you realize that Tartarus is stuck behind the Bane Knights, out of range of the target, and if you activate the Knights to move out of the way, you can't charge with them once Tartarus has cursed... No worries, Darragh Wrathe will solve it: Death Ride the Knights out of the way, so Tartarus can go do his job and they can follow up with the charge at MAT8 and increased range.
- Didn't kill the Target you charged? ... and now your own models are blocking access to it for you last couple of units/models... Darragh Wrathe will solve it, and shuffle a few guys around, allowing you to charge the target with another couple of models to finish the job.
- Annoying solo giving you a hard time... Darragh has 18" threat on his Hellfire, and can move 5" away after the deed is done. He can also charge one if it got close, with 13" threat mind you, then Hellfire another one with Battle Wizard, then return to shield something. Incorporeal models or not, Darragh Wrathe will solve your issue with non-stealth solo annoyances.
- Beyond Death... it's great with casters like Venethrax and Terminus, but it can actually save a lot of lives if used offensively, though that will cost Darragh his life. Park him up front, and opposing models will have to gun him down first.
- Assassination: When eGaspy pops his feat, you can add an inch to their range as well, and spread them out a bit to gain some better vectors on the charges. This has caught a couple of players unaware.