This comes as part of the missions. You have to go to Cirta's Temple. Dennis tells you to go to get some 'power'. Not much to it. Turn up, walk in, meet the young lady and get given a druggy drink - most of this is without any interaction. You then slowly follow a man in a white suit off a beach into a hut then across a rope bridge (all whilst high) up to an altar. Press X to get the dagger, come round to Dennis tattooing your arm a bit more and when you leave the temple you get the Achievement and new skills unlocked. By this point if you have previously used all the early skills you might have some skill points left over to use.