rain, reign, rein

Rain or rain means the drops of water that fall from the sky--
Take an umbrella; it looks like rain.
He got soaked in the rain on the way home.
Do you think it'll rain?

Reign or reign ( rhymes with rain ) means the duration of the rule of a king or queen, or the royal power of ruling the state--
Queen Victoria's reign lasted more than sixty years.
The next sovereign to reign over England will be Prince Charles.

Rein or rein ( rhymes with rain ) means a leather strap used to control a horse or other animal. It may refer to the strap itself  ( a thing ), or to the capacity to control the animal with it--
He checked the horse with a light rein.
Rein in your horse!

Now that you know that, you can say--
"During the reign of the late king, Grandfather helped the prince rein in his horses, which were upset by the rain."

Note--"reins" is the plural ( more than one ) of "rein"-- 
Hold on to the reins, or the horse might bolt.
"Reins" is also an old-fashioned term for a person's kidneys--
Some ancient people believed that the reins were the seat of emotion. 
Note # 2--"arraign" means to make someone appear in court to answer a charge or indictment--
The judge will arraign him on Monday.
Not to be confused with "a rain"--
They were plagued by a rain of frogs.
Or "a rein"--
Try to keep a rein on your temper.
Or "a reign"--
She celebrated a reign of sixty years.