Spellbinder is a pretty underrated item on the champs that can utilize it well

Spellbinder's 2800 gold for 100 AP and 10% MS is already pretty great. And if you max stack it, that's 200 AP and 40% MS for 4 seconds, without Rabbadon or Infernal amplification. Which is near Yi ult level MS boost. MS aside, purely on the stats side..

With Rabbadon's, that's a total** 280 AP **for 4 seconds from Spellbinder and its active. With Rabba and 3 infernals, 328 AP. (356 with Elder's amplification)

For an example of how much of a difference it makes for those that utilize it well, I'll use Evelynn, my main, who is one of the best users (and certainly not the only user) of it. Pre-resists, though resists don't do much against Evelynn, since she's well, Evelynn. Though it varies largely depending on build, some example numbers on the damage you deal from it in the mid-late. Elecrocute scaling not included.

• No Rabba, Spellbinder Active's AP Only: Up to 195 damage on Hate Spike, 2.5% max HP on Empowered Whiplash, 1.5% on Unempowered Whiplash, 75 on Last Caress, 150 on Last Caress against low HP enemies. + 10 on Runic, and 50 on Lichbane.

Total Maximum Bonus Damage from Spellbinder Active: 455 + 2.5% Max HP ( 2 Lichbane procs)

Average Effective Bonus Damage from Spellbinder Active against an enemy with 70-80MR pre-shred, after MR shred and MPen: 374-393 + 2.1%-2.3% Max HP

Pretty hefty amount damage boost for Evelynn for a 4 second period, even without a Deathcap.

• With Rabba, Spellbinder Active's AP Only: Up to 273 damage on Hate Spike, 3.5% max HP on Empowered Whiplash, 2.1% on Unempowered Whiplash, 105 on Last Caress, 210 on Last Caress against low HP enemies. + 14 on Runic, and 70 on Lichbane.

Total Maximum Bonus Damage from Spellbinder Active: 637 + 3.5% Max HP ( 2 Lichbane procs)

Average Effective Bonus Damage from Spellbinder Active against an enemy with 70-80MR pre-shred, after MR shred and MPen: 543-561 + 3.1%-3.3% Max HP

Effectively an entire ult's worth more damage on Evelynn in comparison to the ults of a decent number of champions.

• With Rabba and 3 Infernals, Spellbinder Active's AP Only: Up to 320 damage on Hate Spike, 4.1% max HP on Empowered Whiplash, 2.46% on Unempowered Whiplash, 123 on Last Caress, 246 on Last Caress against low HP enemies. + 16 on Runic, and 82 on Lichbane.

Total Maximum Bonus Damage from Spellbinder Active: 746 + 4.1% Max HP ( 2 Lichbane procs)

Average Effective Bonus Damage from Spellbinder Active against a squishy with 70-80MR pre-shred, after resistance shredding: 689-711 + 3.7%-3.9% Max HP

Very unlikely to get this much power out of Spellbinder on Evelynn, as getting triple Infernals as any champ without the enemy surrendering is rather.. rare.

• With Rabba and 3 Infernals and Elder, Spellbinder Active's AP Only: Up to 358 damage on Hate Spike, 4.2% max HP on Empowered Whiplash, 2.46% on Unempowered Whiplash, 123 on Last Caress, 246 on Last Caress against low HP enemies. + 18 on Runic, and 92 on Lichbane.

Total Maximum Bonus Damage from Spellbinder Active: 835 + 4.2% Max HP ( 2 Lichbane procs)

Average Effective Bonus Damage from Spellbinder Active against a squishy with 70-80MR pre-shred, after resistance shredding: 771-796 + 3.7%-3.9% Max HP

And this much is disgustingly huge, but this is highly unlikely. The likelihood of getting 3 Infernals and Elder without the enemy surrendering is.. Well, once in a blue moon. So yeah, for those champions that can really utilize spellbinder efficiently, though not as gigantic an amount of damage as Evelynn can get out of it, they still gain a ton of damage potential and mobility for a short period.

TLDR: Tons of Damage and huge chunk mobility for a short period of time on the champs that can use it well.