Each legendary spell you cast that's one or more colors has web-slinging . (You may cast a spell for its web-slinging cost if you also return a tapped creature you control to its owner's hand.)
Cast Peter Parker early, generate a Spider blocker, then transform him into a vigilant attacker once you have an engine going. The real payoff is the Amazing Spider-Man side: every legendary spell you cast gains web-slinging , letting you slam expensive legendary creatures and planeswalkers for a fixed three mana by bouncing a tapped creature you control. You snowball card and board advantage by recasting bounced legends and looping ETB value.
Massive cost reduction on legendary bombs, often turning 6-7 drops into 3-mana plays
Built-in ETB token plus enables repeated ETB abuse via tapped-creature bounces
Vigilance after transforming lets him attack and block with a Spider token wall
Bant's deep legendary pool gives nearly endless web-slinging targets
Transform is sorcery-speed and costs three pips, slowing early tempo
Web-slinging requires a tapped creature, so it can be awkward before you attack or have utility tappers
Removal-heavy decks blunt him since he wants to commit creatures to the board
No inherent protection; a cheap kill spell sets the whole engine back
A premium legendary draw engine that's cheap to deploy under web-slinging.
Tighten the curve around impactful legendary creatures and planeswalkers so web-slinging always has a target, and add flicker enablers (Brago, Emiel, Roon, Conjurer's Closet) to maximize ETB loops. Improve fixing with Bant duals and ramp like Smothering Tithe and Esper Sentinel to power out the engine, and slot in protection (Heroic Intervention, Teferi's Protection) to keep Peter alive.