Harm Touch is an innate ability to inflict a single massive attack once every 72 minutes. From your inventory screen, select the Alternate Experience tab, go to Class Abilities and you'll see it there. You can click on it and then click on the "Make Hotkey" button to make a hot key for the skill and add it to your bar.
Although it's listed under the Alternate Experience tabs, you can use Harm Touch from level one onwards. At level 51+ you can start adding AAXP points to it to make it inflict more damage.
Note that Harm Touch is technically resistable (I've resisted many a mob's Harm Touch) but I've never had it resisted when I've used it. I'm sure it's possible, especially if you tried to HT a mob much higher in level than you but it's not something that should typically be an issue.
Edit: Curse you, Yther! Edited, Apr 4th 2009 1:33am by Jophiel