Transformation is Kayn's core mechanic; aim to transform quickly. In the current meta, Darkin (Rhaast) is significantly stronger than Shadow Assassin and generally favorable regardless of enemy composition.
Once transformed into Darkin, Kayn gains a massive power spike. Utilize his percentage health damage, immense lifesteal, and wall-walking mobility to dominate teamfights and close out games rapidly.
As a jungler, Kayn excels at clearing camps and objectives like Grubs with his AoE skills and unique gank paths through walls. Avoid playing him in the top lane, and play cautiously during his vulnerable pre-transformation phase.