Seismic Slam is a Secondary Barbarian skill unlocked at Level 12, which causes a powerful shockwave to erupt from the Barbarian, knocking enemies caught in the shockwave back. It's a signature skill of the Barbarian.
The following is a very quick summary. See the Seismic Slam 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.
When launching this skill, the barbarian lifts one foot and brings it down with his weapon, slamming them into the ground and sending a wave of energy forward, dealing heavy damage to anything directly in front of him. Multiple monsters can be hit by each use, and the ground is torn and fractured by this skill.
Seismic Slam is a strong and fun skill to use, causing damage to the environment as well as to enemies. The skill can cause wooden ornaments to be destroyed or can cause walls to collapse. This skill should be aimed to hit multiple enemies at once, as the damage is good and will keep the fury rolling in for the use of other skills.
This skill was first skill we made for Barb, and one of the first we did in the game. Getting it right took longer than it did with most skills. We did many iterations. Animation especially. Had to make barb feel physically powerful. Traditionally we associate big bold movements with power. But if you want a game that feels fast and responsive, you want the movements to be really fast. Getting balance of fast movements and powerful feel. It's a big challenge in many ways to get that animation right.
The Seismic Slam from BlizzCon 2008 only did 100% weapon damage on rank one, but was increased to 120% in the BlizzCon 2009 demo. In every gameplay demo so far, this skill has been too high level to play.
Skill tiers were removed for the July 2011 Press Event, allowing the skills to unlock steadily from levels 1 to 30. Consequently, all skill levels were adjusted and Seismic Slam became a level 18 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.
Seismic Slam was bumped to level 19 for the beta. With the Patch 13 changes, it was classified as a Secondary skill and moved to level 20. For Beta Patch 14, it was dropped down to level 12, allowing it to be played for the first time.