The Spicy Challenge line up consists of four sandwiches in all; three are new with one yet to be revealed.
At Level 1 (the mildest level), there's the new Sizzling Citrus chicken sandwich featuring a green chili lime sauce with crispy-fried chicken filet and whole leaf lettuce on a cornmeal bun.
One level up on Level 2, there's the regular menu McSpicy chicken sandwich (which I tried a while ago in Taiwan, where it's called the "Spicy Chicken Filet Burger"). It features a spicy, crispy-fried chicken thigh filet, with lettuce and mayo on a sesame seed bun. There's also a Double McSpicy (also on the regular menu), which adds on a second thigh filet.
Level 3 comes with a warning that it is "Very Spicy" and features the Flaming Green Curry chicken sandwich. It offers a Thai-influenced green curry (think coconut milk and lemongrass) with a green curry chicken thigh filet, leaf lettuce, and a cornmeal bun.
At Level 4, is a yet unrevealed sandwich that is meant to be the hottest of the bunch. I'll be sure to update regarding it as details are released. Update 5/2/14: Level 4 is the Szechuan-inspired Fiery Mala sandwich:
Along with the Spicy Challenge, McDonald's is also featuring a couple of other new items: Smokey Drumlettes and Yuzu McFizz.
The Smokey Drumlettes feature chicken wing drumlettes marinated in a blend of smoky spice including black pepper and paprika along with onion and garlic.
The Yuzu McFizz is a soda flavored with yuzu, a citrus fruit that's used like a lemon but tastes more like a cross between a grapefruit and mandarin orange. Unlike a grapefruit or mandarin orange, you don't typically peel and eat it.