So i was wondering and this possibly has been asked but i digress can someone explain if The First Sliver cascade effect works with the commander cost effect or no?
If you cast a spell "without paying its mana cost" due to cascade, that doesn't affect any additional costs that may or must be paid (C.R. 118.9, 118.9d, 702.35a). Generally, all that differs when paying for a spell due to cascade is that the spell's mana cost is not paid (C.R. 702.85a).
However, note that if you cast your commander due to cascade, you don't pay any "commander tax", since the commander is being cast from exile, not from the command zone (C.R. 702.85a, 903.8).
I think the question probably was whether commander tax changes a commander's converted mana cost for the purpose of cascade (if I'm wrong, peteroupc's post covers the rest). The answer is no. A card's converted mana cost usually is its printed mana cost, i.e. what's printed in the upper corner of the card, converted to a number, and the only way that number may vary is if there's an X in the mana cost (the value chosen for X counts towards converted mana cost while the card is a spell on the stack, and counts as 0 everywhere else). Alternative costs, cost reductions, and additional costs such as the commander tax don't change a card's converted mana cost. So your commander's converted mana cost never changes unless there's an X in its mana cost. If The First Sliver is your commander, its converted mana cost is always 5 regardless of commander tax, and when you cast it, it will always cascade into a card with converted mana cost 4 or lower.
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So i was wondering and this possibly has been asked but i digress can someone explain if The First Sliver cascade effect works with the commander cost effect or no?
"the commander cost effect" is an ambiguous expression. The fact of paying additional mana for casting your commander from the command zone, is a rule, not an effect. The cost added itself is an additional cost levied in the situation of using that rule which lets you cast a card from the command zone.
"works with" is also ambiguous. Everything mentioned exists and applies together - nothing overrides anything else mentioned or causes you to ignore one of them.
If your commander is a Sliver, or The First Sliver, then casting it in any way, even from the command zone, triggers cascade. If your commander is revealed through cascade, this situation is not where you cast your commander from the command zone. As MadMage says, the commander tax does not change the converted mana cost (soon to be: mana value) of the card. Converted Mana Cost is derived from the Mana Cost -> the attribute in the top right corner -> and that attribute is never changed by anything unless there is an in it or the card becomes a copy of something. The converted mana cost of something with Mana Cost WUBRG is 5, always.
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However, note that if you cast your commander due to cascade, you don't pay any "commander tax", since the commander is being cast from exile, not from the command zone (C.R. 702.85a, 903.8).
See also this thread and this thread.
EDIT (Nov. 8): Edited rule citations, including because some rules were renumbered in the meantime.
"the commander cost effect" is an ambiguous expression. The fact of paying additional mana for casting your commander from the command zone, is a rule, not an effect. The cost added itself is an additional cost levied in the situation of using that rule which lets you cast a card from the command zone.
"works with" is also ambiguous. Everything mentioned exists and applies together - nothing overrides anything else mentioned or causes you to ignore one of them.
If your commander is a Sliver, or The First Sliver, then casting it in any way, even from the command zone, triggers cascade. If your commander is revealed through cascade, this situation is not where you cast your commander from the command zone. As MadMage says, the commander tax does not change the converted mana cost (soon to be: mana value) of the card. Converted Mana Cost is derived from the Mana Cost -> the attribute in the top right corner -> and that attribute is never changed by anything unless there is an in it or the card becomes a copy of something. The converted mana cost of something with Mana Cost WUBRG is 5, always.
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