Ray of Frost is a Secondary Wizard skill unlocked at Level 2. It fires a straight beam of cold that chills and damages a single enemy, while draining Arcane Power steadily.
This skill is often compared to Disintegrate as both fire a straight beam of energy, but they are quite different in function. Both have a high resource drain while active, but Ray of Frost does not pierce, making it much less damaging against multiple enemies. It does do good damage to a single target though, and the chilling effect slows enemies, making it helpful to land more hits on an approaching foe.
The skill is fun to use and very effective against single targets, with good damage and the slowing property. However the Arcane Power drain is quite high, and thus the damage per second is much lower than a number of other early-game Wizard skills.
This skill targets much like Disintegrate, but trades the piercing of that skill for a slowing cold effect. This makes Ray of Frost excellent against single targets, but much less damaging on the whole, especially against large groups of enemies.
Ray of Frost was first seen at BlizzCon 2008, but only in the gameplay movie; the Wizard used it against the trap laid out by the Skeleton King. It was available in the gameplay demo at BlizzCon 2009, but was unfortunately too high level to try out by the fans.
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 Ray of Frost became a level 21 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.
Ray of Frost remained at level 21 when the beta started. With the Patch 13 changes, it was classified as a Secondary skill and dropped all the way down to level 5, giving players their first opportunity to try it out. For March's Beta Patch 14, it was dropped even further to level 2, but its damage output was increased slightly.