I haven't kept up on Dragonlance lore very well throughout the years. I am still seeing quite a bit of art on various searches for Raistlin.
Why is he often depicted as weak and/or ill? Last I really remember he defeated a deity. Someone that powerful shouldn't look so weak...
Raistlin was born like this, with a very weak body. Because of his disability, he focused on developing his mind, becoming one of the most powerful wizard that ever existed in the Dragonlance Universe. But with all that magic, his body remained weak.
The Wikipedia entry says it all.
Raistlin Majere and his twin brother Caramon were born in the town of Solace, Abanasinia, to a woodcutter named Gilon Majere and his wife, Rosamun (who, because she possessed magical abilities but was forced to suppress it by her upbringing, had a habit of slipping into uncontrollable trances). They were preceded by an older half-sister, Kitiara uth Matar, the product of Rosamun’s first marriage to a purported ex-Solamnic Knight, Gregor uth Matar. Raistlin was born sickly and on the verge of death. In fact, the midwife felt that the baby Raistlin should be allowed to die as an act of mercy. Only due to Kitiara’s stubborn refusal to let him die (and subsequent treatment and care) did he finally overcome the infantile ailment and live.
Possibly as a result of this early illness, Raistlin was always a sickly child, but possessed a fierce intellect, in stark contrast to his slow-thinking, but not unintelligent, physically stronger twin. In fact, according to the common lore of the world of Krynn (supported by statements made by the god, Paladine, in the closing chapters of Dragons of Spring Dawning), identical twins were considered to be one person split in two. For this reason, Raistlin believed he had been given the mind, and Caramon the body.
And as stated by Obsidian Phoenix, the Test passed by Raistlin to become a Wizard left him in a condition even worse. As a compensation, he got awarded with a powerful magic staff (the Staff of Magius), and a recipe for a herbal tea that ease his pain.
Raistlin had a weak body from the beginning. As mentioned in the previous answer, he nearly died shortly after being born, but was nursed to life by his half-sister Kitiara.
Furthermore, during the test, Raistlin made a bargain with the old lich (undead wizard)1 Fistandantilus. Raistlin got even weaker, as Fistandantilus exchanged part of Raistlin's life force for his help:
And numerous other occasions.
Thus Raistlin became weak, both due to the injuries received in the Test and due to Fistandantilus's drain imposed on his body.
In The Soulforge it is revealed,
that it was not the test that shattered Raistlin's body, but his bargain with Fistandantilus did, because the lich drained his life force in order to sustain himself. After the test, Raistlin forgot about his bargain with Fistandantilus. The High Wizards Par-Salian (head of the Conclave) and Antimodes (Raistlin's sponsor in his studies) decided not to tell him, because he could only take advantage of the knowledge if he learned it himself. Thus Raistlin believed that he was crippled in the Test and blamed the Conclave for that.
In the events of Dragons of the Hourglass Mage, it is revealed that
Raistlin first trapped Fistandantilus within the Dragon Orb, then, in their final encounter, killed Fistandantilus by draining the life force with the ruby pendant out of the body Fistandantilus was given by Takhisis.
As a result, Raistlin became more powerful in mind and in body (he stopped coughing and nearly fainting after each spell). This is the Raistlin we see at the end of The Dragons Of The Spring Dawning, where he defeats a number of the Dark Queen's minions and Caramon, wounded, leans on him for support.
In the Legends trilogy,
Raistlin travelled back in time and defeated Fistandantilus again, gaining his life force anew in a similar fashion.
By the time he is about to enter the Abyss, Raistlin is no longer weak and frail.
Raistlin is born weak, but is also very intelligent. Raistlin heads to the Tower and takes the Test. Due to the incidents in the Test and also the intervention of Fistandantilus, Raistlin is changed, to come out of the Test, Gold Skinned with HourGlass Eyes, and an all body "illness". Actually the illness is his "contract" with Fistandantilus. Raistlin gets his staff and the tea he drinks from the Conclave of the Tower.
Hence why He is seen as weak bodied. Later Raistlin takes in Fistandantilus's essence or soul or spirit. Later Raistlin Goes back in time and learns from then takes Fistandantilus's essence or soul or spirit again in his younger form.
At this point Raistlin is anything but weak in any way, and on top of that he changes from a Red Robed wizard to a Black Robe wizard and goes slightly Maniacal on top of being Very Arrogant and Egotistical.
Basically at the time he goes to the Abyss, Raistlin is awesome and damn near godly in power and ability.
