Entwine [cost] (Choose both if you pay the entwine cost.)|
21% 21% 21% 21% 16%
Entwine is a keyword ability appearing on modal instants and sorceries, ie. ones with the "Choose one —" clause.
As you cast a spell with entwine, if you additionally pay the entwine cost, instead of choosing only one of the modes of the spell, you choose both.
Spells with entwine were featured in the Mirrodin block.  Mark Rosewater came up with the mechanic in his sleep. 
From the Comprehensive Rules (Ravnica Allegiance (January 25, 2019))
- 702.41. Entwine
- 702.41a Entwine is a static ability of modal spells (see rule 700.2) that functions while the spell is on the stack. "Entwine [cost]" means "You may choose all modes of this spell instead of just one. If you do, you pay an additional [cost]." Using the entwine ability follows the rules for choosing modes and paying additional costs in rules 601.2b and 601.2f-h.
- 702.41b If the entwine cost was paid, follow the text of each of the modes in the order written on the card when the spell resolves.
From the glossary of the Comprehensive Rules (Ravnica Allegiance (January 25, 2019))
- A keyword ability that lets a player choose all modes for a spell rather than just one. See rule 702.41, "Entwine."
- You decide whether you're paying the entwine cost as you play the spell.
- If you choose not to pay the entwine cost, you can choose only one mode. If you choose to pay the entwine cost, you choose both modes.
- If you pay the entwine cost, choose targets for both modes as you play the spell. When the spell resolves, follow the instructions in the order they're written.
- Betrayal of Flesh -
Choose one —
• Destroy target creature.
• Return target creature card from your graveyard to the battlefield.
Entwine — Sacrifice three lands. (Choose both if you pay the entwine cost.)