Slark + Viper

Why this works

Picking these two cores together often creates a team that's very good at running down a single hero. Viper brings extreme slows with his poisons and ultimate, letting Slark stick to a target to build Essence Shift stacks. Nethertoxin breaking a key passive on that target is also a significant help. The major downside is the complete lack of hard lockdown. With only slows and a leash from Pounce between them, enemies with a Force Staff, mobility spell, or teleport can often get away. The duo provides a lot of damage and control over one person, but no way to truly pin them down.

This means your mid-game pickoffs can feel very effective, but larger teamfights are less certain. Slark and Viper can excel at isolating and bursting one hero before a fight, but they struggle to deal with a coordinated team that can save its allies. Without a stun to interrupt a key channel or stop a hero from escaping, the enemy can often reposition and punish your single-minded aggression. This forces the rest of your team to draft heavy control, making your strategy predictable and easy to play around if the enemy has good saves and mobility.