Vertical circular motion
Let's take it from the top.
KE = (1/2)mv2; PE = mgh
The total mechanical energy is constant, KE + PE = Constant
Therefore when the pendulum goes higher, two things happen PE increases because h increases and KE decreases to keep the sum KE + PE constant.
But if KE decreases, v must also decrease.
Now in terms of angular velocity ω, v = ω r.
Here, r is the length of the pendulum. This stays constant as the pendulum swings back and forth.
Looking at v = ω r, if v decreases and r stays the same, what happens to ω?