Monastery Swiftspear

• WOTC Official Reminder Textofficial rules
  Prowess — Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.

• WWizards of the Coast2014-09-20
  Official Ruling
  Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
• WWizards of the Coast2014-09-20
  Official Ruling
  Once it triggers, prowess isn't connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
• WWizards of the Coast2014-09-20
  Official Ruling
  Any spell you cast that doesn't have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won't cause prowess to trigger. Playing a land also won't cause prowess to trigger.
• WWizards of the Coast2021-03-19
  Official Ruling
  An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Rulings via Scryfall

• WO
  Wizards of the Coast2023-12-04

  Monastery Swiftspear was banned in pauper on December 4, 2023. Monastery Swiftspear is now banned in Pauper. The Pauper Format Panel found that it kicked off the huge influx of red, synergizes with 'bottle two' spells, dodges two-toughness removal, and generally makes for the most explosive draws, contributing to the format's speed and polarization. Source: https://magic.wizards.com/en/news/announcements/explanation-of-pauper-bans-for-december-4-2023