It's always been strange to me that Ishamael visited Lews Therin in the prologue of the first book. What did he want to achieve?

Well Ishamael certainly had his reasons. Lews Therin was his arch-enemy and he humiliated him before the Hall of the Servants, and defeated him in combat in front of the gates of Paaren Disen. However, for Ishamael to Travel to Lews Therin's house where his whole family lives would require suicidal courage. Bear in mind that he was unaware of Lews Therin's mass murder of his family.

I think that Ishamael returned to convert Lews Therin to the Shadow.

When Ishamael arrives he says, 'Lord of the Morning, I have come for you," which may mean that he's come to kill him, or merely to turn him to the Shadow. Later he gets irritated that Lews Therin's mind is tainted, and he decides to Heal him saying that his weak Healing will 'suit his purposes'.

As a madman, Lews Therin would be of little use to Ishamael and couldn't be reasoned with. It would also make it impossible to convert him to the Shadow. He then makes Lews Therin sane and shows him what he's done to Ilyena and his family. This is the perfect catalyst to make LTT desperate enough to do anything - and Ishamael says that he can have Ilyena back if he joins the Shadow.

It seems crazy to think that Ishamael would try this tired tactic of his after the Strike at Shayol Ghul, but it kind of makes sense. Ishamael was cut off from the Dark One, and from his allies (the other Forsaken). The only real hope he had was to turn Lews Therin, who would help him rupture the seals or give him the knowledge to do so. Either way, he was surprised and angry that Lews Therin was already insane, and he rectified it for his own purposes. He was not counting on Lews Therin's suicide, which effectively ended his short-lived plot.