The mace is an iLevel 226 item and has a DPS output of 171.5 -- making it on par with the very best 1H weapons out of 25-man Naxx. Full stats are in the picture above.
It looks like, well, a tankard. Just like the other glasses that are in the game. I don't necessarily get why it's "O' Terror," that would sound like it belongs more with the upcoming Hallow's End than Brewfest, but that's just me. Either way it's quite a nice mace that's going to make a lot of Enhancement Shamans happy.
It can also make your wallet a little fatter if you're lucky enough to get it (it does have a low drop rate). They're selling for between 4k to 5k gold. I would suspect that'll decrease substantially in the next few days however. If you really want it and don't want to spend that kind of gold, Brewfest is running for a while still -- you'll have until October 3rd to get your hands on one.
For more Brewfest information, check out our Guide to the 2009 Brewfest Achievements and the rest of the updated level 80 Direbrew loot.