Doom + Outworld Destroyer

Why this works

Setting up a kill with Doom seems like it would be a great way to let Outworld Destroyer unload, but their biggest spells actively work against each other. Sanity's Eclipse does its best damage against enemies who have spent a lot of their mana. A target suffering from Doom is muted and unable to cast spells, which means their mana pool stays full. Using both ultimates on the same hero makes one of them much less effective, and you’re usually forced to choose which one gets its full value while the other is partially wasted.

This conflict of interest often forces Doom and Outworld Destroyer to focus on different enemies in a teamfight. Doom will try to shut down the most dangerous spellcaster, but that target is now the worst one for OD to hit with his ultimate. This either splits the team's damage on two different heroes or forces OD to use his biggest teamfight spell on a lower priority target. The whole sequence becomes awkward and less efficient than if they could simply combine their full power onto a single, important enemy.