NOTES: Long, pointed, deep-red dried guajillo chiles are available in Mexican markets. If you can't find them, substitute dried ancho chiles. Tamarind is a sweet-sour pulp that comes in many forms; the Indian tamarind concentrate used here is a thick, sticky, dark brown, smooth syrup. Look for it in specialty food stores, Indian markets, or Latino markets. If you can't find it, substitute equal parts molasses and lime juice. You can make the glaze up to a week ahead; chill airtight.
Cut top 1/2 inch off garlic; wrap loosely in foil. Bake in a 350° oven until soft when pressed, about 45 minutes. Let cool, then squeeze garlic from skins into a 3- to 4-quart pan (discard skins). Add chiles, tamarind concentrate, cilantro, 1 cup water, honey, lime juice, pepper, and salt to pan. Bring to a boil over medium-high heat, stirring often; remove from heat and let stand 10 minutes. Pour into a blender or food processor and whirl until smooth.