Any Nurglings included in an army may be set up at the start of the battle if you want, and do not have to be summoned to the battlefield. Note that you can choose to summon Nurglings if you wish, it's just that you don't have to. Nurglings cannot be banished back to the warp.
Building an army? Create, validate and share your list with New Recruit, army builder for Warhammer 40,000 2nd Edition and many more games.
Try the builder