+2: Put a charge counter or a +1/+1 counter on up to one target artifact.
-3: You may put an artifact card from your hand onto the battlefield.
-8: You get an emblem with "Artifacts you control are artifact creatures with base power and toughness 5/5."
See Challenge # 129. Karn and Tezzeret are another odd pairing of planeswalkers: their loyalties are fairly opposite, but their mechanical schtick is very similar (as long as you remember what Karn used to be like before his spark awoke). They particularly align when it comes to animating artifacts: Tezz makes things 5/5s, while Karn makes them CMC/CMC, but 5/5s works better for an emblem.
I really like the +2. It's something I'd definitely like to see printed on a card in the future.