𝐎𝐧𝐞 𝐝𝐚𝐲 𝐡𝐞 𝐰𝐚𝐬 𝐚𝐥𝐦𝐨𝐭 𝐝𝐢𝐞𝐝 𝐚𝐧𝐝 𝐭𝐡𝐞𝐫𝐞𝐬 𝐰𝐡𝐞𝐫𝐞 𝐡𝐞 𝐦𝐞𝐭 𝐒𝐮𝐤𝐮𝐧𝐚. 𝐒𝐮𝐤𝐮𝐧𝐚 𝐡𝐞𝐚𝐥𝐞𝐝 𝐡𝐢𝐦 𝐚𝐧𝐝 𝐚𝐬𝐤𝐞𝐝 𝐨𝐟 𝐡𝐞 𝐧𝐞𝐞𝐝𝐞𝐝 𝐡𝐞𝐥𝐩. Sukuna also made Rihan immortal he could still die but his healing process would be faster and so would his reverse curse technique. 𝐖𝐢𝐭h𝐨𝐮𝐭 𝐡𝐞𝐬𝐢𝐭𝐚𝐢𝐨𝐧 𝐑𝐢𝐡𝐚𝐧 𝐚𝐜𝐜𝐩𝐭𝐞𝐝 𝐢𝐭. 𝐒𝐮𝐤𝐮𝐧𝐚 𝐦𝐚𝐝𝐞 𝐑𝐢𝐡𝐚𝐧 𝐚𝐧𝐝 𝐜𝐡𝐚𝐫𝐦 𝐭𝐡𝐚𝐭 𝐜𝐚𝐧 𝐛𝐥𝐨𝐜𝐤 𝐚𝐧𝐝 𝐭𝐢𝐦𝐞 𝐜𝐮𝐫𝐬𝐞 𝐭𝐞𝐜𝐡𝐪𝐢𝐮𝐧𝐞 𝐚𝐧𝐝 𝐢𝐧 𝐫𝐞𝐭𝐮𝐫𝐧 𝐡𝐢𝐬 𝐜𝐥𝐚𝐧 𝐰𝐢𝐥𝐥 𝐬𝐞𝐫𝐯𝐞 𝐚𝐧𝐝 𝐝𝐨 𝐰𝐡𝐚𝐭 𝐞𝐯𝐞𝐫 𝐡𝐞 𝐭𝐞𝐥𝐥𝐬 𝐭𝐡𝐞𝐦 𝐝𝐨 𝐭𝐨. In the Kagami clan it was only Rihan who was offered that offer only Rihan in his clan lived forever and he's is the founder of the Kagami clan. Since Rihan made a deal with Sukuna his clan serves him for ever.
One day the Kagami clan attacked the Tsukikawa clan. The Gojo clan came in time to safe them and the Kagami clan retreated. Since then the Gojo clan and the Tsukikawa clan have been close.
The gods did not gift the two sibling as to become rivals. The Gave them these gifts the bring peace to the world and defeat Sukuna but Rihan saw it in a different way
(So the main curse technique is time curse technique but anything that has a connection to the moon Setsuna is able to use to the Tsukikawa clan is time curse techniquesa