Which sage cursed King Pandu?

The sage who cursed King Pandu was Kindama.

While hunting in the forest, Pandu shot at what he believed was a pair of deer mating. The animals were actually the sage Kindama and his wife, who had assumed deer forms so they could be intimate away from human society. Mortally wounded, Kindama revealed his identity and cursed Pandu: if Pandu ever approached one of his wives with sexual desire, he would die immediately.

The curse ended Pandu’s ordinary prospects as a king and father. Overcome with remorse, he renounced active kingship and retired to the forest with his wives, Kunti and Madri. Because he could not safely conceive children, Kunti used a mantra granted by the sage Durvasa to invoke deities. Through that boon, Yudhishthira, Bhima and Arjuna were born; Madri invoked the Ashvin twins to become the mother of Nakula and Sahadeva.

The curse also shapes Pandu’s death. During his forest life, he was overcome by desire for Madri and embraced her, whereupon the curse took effect. Madri died with him, while Kunti raised the five Pandavas. Kindama is therefore not merely a minor character: his curse explains both the Pandavas’ divine births and the succession crisis that follows.