Yes and no. You cannot ever voluntarily choose to discard cards from hand or otherwise send cards to the graveyard - only if allowed to do so by cost, effect, or game mechanic.
If you have more than 6 cards in hand at the end of your end phase, then you must discard down to 6. So technically this is a discard without being told to do so by an effect. But if the question was asking can you voluntarily discard 'for no reason', the answer is no.
No you can't discard unless instructed to by a card or effect, or you have exceeded your hand size limit and are at the end of your turn.
Since dark illusion is a monster card it is destroyed by raigeki without it's effect coming into play.Same as if a face down effect monster were to be destroyed by raigeki the effect of the card could not be used so any face down effect cards or any effect cards on your side of the field are rendered useless.
Magician of faith is banned because other cards like monster reincarnation require a discard to work.
Only if the card specifically says it can summon a Ritual Monster, like Ritual Foregone. Otherwise, you can't do it, cards like A Hero Emerges, or Monster Gate, cannot special summon a Ritual Monster.Also if you summon the Ritual Monster properly, by Ritual Summon, and it is then sent to the graveyard for whatever reason, then you may bring it back to the field using cards like Monster Reborn. However you cannot simply discard a Ritual Monster from hand and try using Monster Reborn straight away, they are a form of special summon only monster, simply by being Ritual Monsters.
then all cards on the field are destroyed unless the union monster has an effect that says that it can be destroyed instead of the equipped monster
You cannot simply choose to discard a card from hand, voluntarily, or send cards from the field. You can only do so when allowed to do so for a cost, or instructed to by an effect or game mechanic.
In Magic: The Gathering, you discard cards when you have more than seven cards in your hand at the end of your turn.
You can't ever just discard cards for no reason, you can only do so if instructed to by a card effect or a game rule, such as the one that lets you replace one Field Spell Card with another.
Yes, in Monopoly Deal, you can discard properties by playing them as part of a set or by using action cards that allow you to discard cards from your hand.
In general it means all effects, be it the resolving effect of a spell, trap or monster, or the reusable effect of a continuous spell or trap.
You cannot end the turn with more than 6 cards in hand, so if you do your usual end of turn discard, and somehow you end up with more cards in hand (such as through Super Rejuvenation) then you will still have to discard down to 6 again.
It will 'resolve without effect' because its target has gone.