Both Uses of

phase
in
The Lords of Discipline

At least under the first phases of torture.†
Chpt 3.27 *

phases = periods of time or stages during the development of something

Now, the outside world could not enter fourth battalion to witness the second phase of Hell Night.†
Chpt 2.16

Definitions:

(1) (phase as in: a critical phase)
a period of time or a stage during the development of something
(2) (phase as in: she's going through a phase)
a period of time when someone exhibits unusual behavior
(3) (phase as in: phased out over three years)
make a change gradually (phase in = gradually add; phase out = gradually removed)
(4) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More specialized and less common senses of the word are found in astronomy, physics, chemistry, and zoology. Consult a comprehensive dictionary if you wish to see those. All senses have to do with stages or periods of time.