I've just defeated Ellmac. Before that Bahamut yes, Sakit and obviously Amphisbaena. I haven't found the Ankh in the inferno cavern yet, nor the jewel. I also have access to the Chamber of Extinction, but I can't see anything in there.
IIRC, the jewel for Inferno Cavern is hidden in its backside area, the Tower of Ruin (which has no boss of its own). Or you can use a jewel from any other area, and I recommend doing so if you have one, as beating Viy is essential to unlocking many of the late-game areas.
As for the Ankh, IIRC there is a way to make a pedestal appear at the bottom of that long column of lava that runs alongside that chimney in the Inverno Cavern with all the eyes spouting fire at regular intervals. Placing a weight on the pedestal makes the Ankh appear (and there's something you have to do to get into the room the Ankh is in, too) but I don't remember the details.
And although the chamber of extinction doesn't get lit permanently until later, you can still do a fair bit in there. Note that firing off a flare into one of the black gemstones you find in there will light up the level for a short period of time - just remember to take note of where the gemstone is in each room as you enter it, or you'll have to hunt around for it again.