Arcane Orb is a Secondary Wizard skill unlocked at Level 5, which explodes on touch, delivering Arcane damage.
The Wizard summons a glowing orb that upon touch will deal great damage to all foes and objects in its range.
Skill Rune Effects
The following is a very quick summary. See the Arcane Orb rune effects page for a more thorough description of rune effects, or any of the individual rune pages for numbers, screenshots, blue quotes, and much more.
<skill class="Wizard">Arcane Orb</skill>
<skill class="Wizard" rune="Obliteration">Arcane Orb</skill>
<skill class="Wizard" rune="Arcane Orbit">Arcane Orb</skill>
<skill class="Wizard" rune="Arcane Nova">Arcane Orb</skill>
<skill class="Wizard" rune="Tap the Source">Arcane Orb</skill>
<skill class="Wizard" rune="Celestial Orb">Arcane Orb</skill>
A glowing orb of energy travels about 10 yards, before bursting in a nova of destruction. The spell looks something like the D2 Sorceress' Frozen Orb, but without the spray of ice spears as it flies across the room. It works like the Fireball of Diablo I.
This one very quickly became Flux's skill of choice for ranged attacks. It dealt substantial damage with a big AoE splash, and had a reasonable mana cost. He is dubious about the listed 1-2 AoE damage, since a few of these into a group of enemies left no survivors.
The following Wizard passives provide a benefit to Arcane Orb, runed or not:
Arcane Orb was first shown at BlizzCon 2008 when the Wizard debuted, on the Arcane Skill Tree at Tier III.
By BlizzCon 2009, the damage had been ever so slightly tuned down, and so had mana cost. Flux talked about a missed opportunity:
There was one scenario area in the desert (shown in the gameplay movie) where about five Dune Dervishes burst out of the ground at once. I didn’t think to try it at the time, but in retrospect I wish I could have herded the Dervishes into a cluster, and then tried throwing in some Arcane Orbs. Just to see if my Orbs would have been bounced around like pachinko balls.
Arcane Orb was available in the BlizzCon 2010 demo, but wasn't one of the Wizard's starting skills. Skill tiers were removed for the July 2011 Press Event, allowing skills to unlock steadily from levels 1 to 30. Consequently, all skill levels were adjusted and Arcane Orb became a level 8 skill.
The Diablo III Beta started in September 2011, and while it ran, all of the skills continued to undergo extensive iteration. Skill changes varied from changing the levels the skills are unlocked, to changes in resource cost, damage, and even function. A few skills were removed altogether, and a few new ones were added. Also, the entire skill system received an overhaul with the Beta Patch 13 update in February 2012, with physical runes replaced by rune effects that automatically unlock at various levels for the different skills.
Arcane Orb remained at level 8 when the beta started. Flux described how to use it effectively:
They do hit with a big boom, and if you get one to the edge of a big pack, it will deal substantial damage. The tricky part about using them (aside from the huge AP cost) is that they are large and move very slowly. This makes it easy to miss. Say you have a big clump of monsters, with several individuals scattered in front. You want the Arcane Orb to hit the big pack, not the solo enemy, but it’s hard not to. In that situation you’d be better off using some other skill, even Magic Missile, to kill the individual before launching your Arcane Orbs into the bigger cluster of targets.
With the Beta Patch 8 of December 2011, Arcane Orb was dropped to level 3. With the Patch 13 changes, it was classified as a Secondary skill and moved to level 2. For March's Beta Patch 14, it was bumped up to level 5, and its damage was nerfed quite a bit.
See the Wizard skill archive for more details on previous versions of Arcane Orb and other Wizard skills.
Or see Arcane Orb (BlizzCon 2008) for the original version of this skill.
In Other Games
Arcane Orb seems to be a spiritual successor to Diablo II's Sorceress skill Frozen Orb, which also features a slow-moving globe floating across the screen, but includes a spray of ice spears.
Arcane Orb can be seen in action on video here:
You can find pictures in the Diablo III screenshot and picture gallery:
Arcane Orb just exploded.
Skill animation sequence.
The maximum range shown thanks to some elevation.
Maximum Arcane Orb range and casting speed demonstration, from the Diablo 3 beta.