The “Development” header is missing, the content is under “Treasure”.
Just below the passage you quote, we see the “Treasure” section, which contains the info you seek:
The remorhaz lair can be explored and plundered if the creatures are killed. The area is a 20-foot-diameter oval chamber with walls of glazed ice. Scattered on the floor are three 500 gp gemstones, fifteen 100 gp gemstones, and one magic item determined by rolling on Magic Item Table C in chapter 7 of the Dungeon Master’s Guide.
[“Development” header should be right here but isn’t]
If the characters slay Wormblod and bring proof of his demise, the couatl emerges from the altar and bestows on each of them a charm of restoration (see the “Supernatural Gifts” section in chapter 7 of the Dungeon Master’s Guide). The couatl then returns to the altar and makes no further attempt to communicate with them.
The section in the question says “if the characters present proof of Wormblod's death as offering to the altar, see ‘Development’”, and the italicized passage above tells what happens if you present proof of Wormblod’s death to the altar.
As for the concubine, the quote given in the question is the only occurrence of the word “concubine” in the entire adventure, so I’m not sure we can find that one. Kuerten postulates in their answer a possible candidate, but descriptions of that person do not seem to line up with “concubine”.