ilhuicac.

See an image that represents ilhuicac in the Visual Lexicon of Aztec Hieroglyphs, ed. Stephanie Wood (Eugene, Ore.: Wired Humanities, 2020-present).

Neologisms:
ilhuicac tetlamachilli = joy in heaven
ilhuicac tlaxcaltzintli = the bread of heaven
Susanne Klaus, Uprooted Christianity: The Preaching of the Christian Doctrine in Mexico, Based on Franciscan Sermons of the 16th Century Written in Nahuatl (Bonn: Bonner Amerikanistische Studien e. V. c/o Seminar für Völkerkunde, Universität Bonn, 1999), 251.

yn elvicac civapili Santa Maria = the heavenly noblewoman Saint Mary
Fray Alonso de Molina, Nahua Confraternities in Early Colonial Mexico: The 1552 Nahuatl Ordinances of fray Alonso de Molina, OFM, ed. and trans., Barry D. Sell (Berkeley: Academy of American Franciscan History, 2002), 136–137.

ilhuicac = in the sky, heaven
Louise M. Burkhart, Holy Wednesday: A Nahua Drama from Early Colonial Mexico (Philadelphia: University of Pennsylvania Press, 1996), 195.

ompa yn elhuicac= there in heaven (Central Mexico ,1552)
Fray Alonso de Molina, Nahua Confraternities in Early Colonial Mexico: The 1552 Nahuatl Ordinances of fray Alonso de Molina, OFM, ed. and trans., Barry D. Sell (Berkeley: Academy of American Franciscan History, 2002), 82–83.

yn yehuatzin ylhuicac ychpochtli = the celestial virgin (San Simón Pochtlan, 1695)
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), Doc. 5, 70–71.

no ihuan niquimotlatlactilia mohtintzitzi quexquih Santos ihuan Santas in onpa motemiltiti Cate in ilhuiCac inic nopan Motlatoltitzinosque = and I also implore all the male and female saints that fill heaven to speak on my behalf (San Pedro Calimaya, Toluca Valley, 1737)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 190.

quinteuma, teteu impan quinma quinteutocac: ic notzaloque, ic tocaiotiloque, teteu ilhuicac vitze = he took them for gods, considered them gods, worshiped them as gods. They were called and given the name of gods who have came from heaven
James Lockhart, We People Here: Nahuatl Accounts of the Conquest of Mexico, Repertorium Columbianum v. 1 (Los Angeles: UCLA Center for Medieval and Renaissance Studies, 1993), 82.

Ilhuicac tetlauhtilli, in Ilhuicac teutenemactli = the fit of heaven (communion) (Juan Bautista, ca. 1599, Mexico City)
Susanne Klaus, Uprooted Christianity: The Preaching of the Christian Doctrine in Mexico, Based on Franciscan Sermons of the 16th Century Written in Nahuatl (Bonn: Bonner Amerikanistische Studien e. V. c/o Seminar für Völkerkunde, Universität Bonn, 1999), 247.

In the Florentine Codex we sometimes see mictlan paired with ilhuicac.

ca mjctlan, ca ylujcac in otontemoc, in otontlachix = for thou hast descended into, thou hast beheld the land of the dead, the heavens (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 32.

njman qujpepetlaoa in piltontli, njman nauhcampa qujiaoa: njman canococuj, ilvicac conjiaoa = Then she uncovered the baby. Then she raised it as an offering in the four directions; then she lifted it up, she raised it as an offering to the heavens (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 205.

ilhuicac tlatouani = señor de cielo
Vidas y bienes olvidados: Testamentos indígenas novohispanos, vol. 1, Testamentos en castellano del siglo XVI y en náhuatl y castellano de Ocotelulco de los siglos XVI y XVII, eds. Teresa Rojas Rabiela, Elsa Leticia Rea López, y Constantino Medina Lima (Mexico: CIESAS, 1999), 232–233.

Campa moyeztica in Theotl? Ilhuicac, tlalticpac, ihuan nohuian [= Dónde está Dios? En el cielo, sobre la tierra, en todos lados]
Antonio Vázquez Gastelu, Arte de lengua mexicana (Puebla de los Angeles, México: Imprenta Nueva de Diego Fernández de León, 1689), 33v.