Escalate is a keyword ability appearing on modal instants and sorceries, ie. ones with the "Choose one or more —" clause. By paying the escalate cost, additional modes may be activated.   The ability was featured in Eldritch Moon. It was designed by Sam Stoddard's development team, and inspired by the old entwine mechanic. 
From the Comprehensive Rules (Ixalan (September 29, 2017))
- 702.119. Escalate
- 702.119a Escalate is a static ability of modal spells (see rule 700.2) that functions while the spell with escalate is on the stack. “Escalate [cost]” means “For each mode you choose beyond the first as you cast this spell, you pay an additional [cost].” Paying a spell’s escalate cost follows the rules for paying additional costs in rules 601.2f–h.
From the glossary of the Comprehensive Rules (Ixalan (September 29, 2017))
- A keyword ability on some modal spells that adds a cost for choosing additional modes. See rule 702.119, “Escalate.”
- Blessed Alliance -
Escalate (Pay this cost for each mode chosen beyond the first.)
Choose one or more —
• Target player gains 4 life.
• Untap up to two target creatures.
• Target opponent sacrifices an attacking creature.
- ↑ Matt Tabak. (June 27, 2016.) “Eldritch Moon Mechanics”, magicthegathering.com, Wizards of the Coast.
- ↑ Sam Stoddard. (July 1, 2016.) “Developing Eldritch Moon”, magicthegathering.com, Wizards of the Coast.
- ↑ Mark Rosewater. (July 4, 2016.) “Over the Moon, Part 2”, magicthegathering.com, Wizards of the Coast.