I still don't get what's with the bloodseeker counter picks slark, it's not gonna work. The thirst passive counter his heal when low and rupture prevents him from moving, but why would he need to move if he can lease on and kill with ult on. The best way would be heroes like viper or meepo, at least have easy burst damage on him so he be dealt with a lot of damage or using viper nethertoxin to remove his passive stacks or regen during ult form, so he can't heal not have any damage, attack speed or armor.