TYPO - Under "Special Rules" the position of the altar should be "f" not "a". Also, the labels for the demon spawns are off. The demons should be reordered to wind (a), night (b), earth (c), flame (d), sun (e), frost (f).
The altar acts like a normal obstacle with hit points (i.e. immune to all negative conditions), therefore the "and immunities" is superfluous and should not be there.