Series and parallel alternating current circuits.

1) It is said that when a multi-pin adapter is used to connect more than one devices, to a single electrical socket, then it may result in a fire-breakout or some kind of disaster. Now why does this happen ? also what are the best ways to connect devices in parallel in this manner safely, without the risk of any accidents ?

There is nothing wrong in using a multi-pin adapter in fact. Besides, all the other connections in the circuit are in parallel.

But then as you said, why is it not safe?
There are two types of three pin plug points: normal three pin plugs and power plugs. Normal plugs are rated at 6A and power plugs at 15A. When using multi-pin adapters, the current drawn by all the devices together might increase beyond the rated value which in turn heats up the plug point or wires and break out a fire! There wont be any kind of damage or accidents as far as the current drawn through a plug and the wires is within the rated value. For instance if we are connecting five mobile chargers in one plug using a multi-pin adapter, that will do little damage, totally safe!

You can use the power plug outlets to do the same job with much more safety. But even then you have to make sure current rating of the 'multi-pin adapter' isn't exceeded!

2) how are devices running on ac power connected in series ? can they be connected in series in the first place ? if so how do they function after this ? for example if a couple of bulbs are connected in series, then will the light output of each bulb be reduced, now that the voltage has been split.

Yes, you are right. The light intensity will be reduced. When two bulbs are in series, the total resistance of that circuit increases. So current flowing through each bulbs are reduced along with the voltage. Both current and voltage are halved for two bulbs of same rating and each bulb will consume 1/4th the power otherwise produced individually.