rhyme - I sound like death when I am alone - Riddle

I sound like death when I am alone,

Though I am no longer made of bone.

With a friend our shapes may range

From the eyes of a serpent to the cars on a train.

But that's just in my basic form.

In other shapes, please be forewarned

Blood will spill and heads will roll

When I'm feeling critical.

Think you know the answer yet?

Maybe you should place a bet.

Answer

You are

Dice.

I sound like death when I am alone,

One die.

Though I am no longer made of bone.

Dice used to be made from bone.

With a friend our shapes may range
From the eyes of a serpent to the cars on a train.

Nicknames for dice rolls: Snake Eyes (1 & 1), Boxcars (6 & 6)

But that's just in my basic form.
In other shapes, please be forewarned
Blood will spill and heads will roll
When I'm feeling critical.

Dice can come in many shapes. Used in D&D and other games to assign hit points. Can get a "critical" hit if you roll the right number.

Think you know the answer yet?
Maybe you should place a bet.

Dice used in casino games.