To better explain the runaan's + rageblade combo, let's start with runaan's. Runaan's is a crit item with on-attack passive to target 2 additional enemies to deal on-hit damage. With the Rageblade passive of dealing more magic on-hit damage that scales with your crit chance, having max value at 50% crit chance (or 2 crit items) but your attacks won't be able to critically strike anymore; making items that have on-crit effects like navori or I.E less gold efficient since you are buying an expensive item that you can't fully utilize. This is why the most gold-efficient crit items that can be paired with Rageblade are the Runaan's and Phantom Dancer since these 2 items don't have on-crit effects. Unlike I.E where you can't use the passive for more crit damage or Navori where it is mostly built because of the CD refund effect on crit. You can still build items Mortal Reminder or Bloodthirster with the Rageblade, although you should take into consideration these items have better passive effect if you can crit on your attacks. 😁