If i am playing Thraximundar and have rooftop storm in play. And say Thrax has died once already. Would i still be able to pay 0 to cast him from command zone or will i still have to pay an additional 2?
I ask because Rooftop storm specifies that may pay 0 where other cards (like the spoiled Geode Golem) says without paying mana costs.
Both paying 0 and casting without pay the mana cost are alternative costs. When calculating how much it costs to actually cast a card you have your cost or alternative cost, then additions and subtractions, then trinisphere but lets ignore that. The commander tax is an additional cost so it applies even if you are paying an alternative cost. Cost reductions do reduce the cost of additional costs. So if you had Jet Medallion and Sapphire Medallion, in addition to the storm then Thraximundar would cost 0 the first time they die then 2 the second time and so forth.
If i am playing Thraximundar and have rooftop storm in play. And say Thrax has died once already. Would i still be able to pay 0 to cast him from command zone or will i still have to pay an additional 2?
I ask because Rooftop storm specifies that may pay 0 where other cards (like the spoiled Geode Golem) says without paying mana costs.
There's little actual difference between "you may pay 0" and "you may cast without paying the mana cost". Both are alternative costs, that replace the regular mana cost. In both cases, additional costs still need to be paid, including the "commander tax".
117.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell's text, or applied to it from another effect, that its controller may pay rather than paying the spell's mana cost. Alternative costs are usually phrased, "You may [action] rather than pay [this object's] mana cost," or "You may cast [this object] without paying its mana cost." Note that some alternative costs are listed in keywords; see rule 702.
117.9d. If an alternative cost is being paid to cast a spell, any additional costs, cost increases, and cost reductions that affect that spell are applied to that alternative cost. (See rule 601.2f.)
See also this thread. In general, if a spell is cast—
by paying an alternative cost (including 0) "rather than [paying]" the spell's mana cost, or
"without paying its mana cost" (which is another form of alternative cost),
that alternative cost does not affect additional costs (such as the so-called "commander tax" under C.R. 903.8) (the reminder text on Geode Golem reminds you of just this point), and neither does it affect cost reductions (C.R. 118.9d, 118.9). This is true regardless of—
whether or not reminder text saying that additional costs must still be paid appears on the card, as is the case for Geode Golem (in any case, reminder text "has no game function" [C.R. 207.2, 207.2a]).
Note that due to the formula for figuring a spell's total cost, the only way to "avoid" required additional costs (at least in part) when casting a spell is if a cost reduction effect reduces some or all of those costs (C.R. 601.2f). One example is found in the emerge ability, which includes an alternative cost but offers a way to reduce the spell's total cost if "you chose to pay this spell's emerge cost" (C.R. 702.118a).
EDIT (Aug. 8): Edited.
EDIT (Sep. 8; Oct. 24): Correctness edit.
EDIT (Aug. 27, 2019): Edited, including because some rules were renumbered with Core Set 2020.
I ask because Rooftop storm specifies that may pay 0 where other cards (like the spoiled Geode Golem) says without paying mana costs.
UB Vela the Night-Clad BUDecklist
WBG Ghave, Guru of Spores GBW
WUBRGThe Ur-DragonWUBRGDecklist
There's little actual difference between "you may pay 0" and "you may cast without paying the mana cost". Both are alternative costs, that replace the regular mana cost. In both cases, additional costs still need to be paid, including the "commander tax".
117.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell's text, or applied to it from another effect, that its controller may pay rather than paying the spell's mana cost. Alternative costs are usually phrased, "You may [action] rather than pay [this object's] mana cost," or "You may cast [this object] without paying its mana cost." Note that some alternative costs are listed in keywords; see rule 702.
117.9d. If an alternative cost is being paid to cast a spell, any additional costs, cost increases, and cost reductions that affect that spell are applied to that alternative cost. (See rule 601.2f.)
Note that due to the formula for figuring a spell's total cost, the only way to "avoid" required additional costs (at least in part) when casting a spell is if a cost reduction effect reduces some or all of those costs (C.R. 601.2f). One example is found in the emerge ability, which includes an alternative cost but offers a way to reduce the spell's total cost if "you chose to pay this spell's emerge cost" (C.R. 702.118a).
EDIT (Aug. 8): Edited.
EDIT (Sep. 8; Oct. 24): Correctness edit.
EDIT (Aug. 27, 2019): Edited, including because some rules were renumbered with Core Set 2020.