One question, the letter of; Rakdos, Lord of Riots his ability says; Creature spells you cast cost 1 less to cast for each 1 life your opponents have lost this turn., Ok my question is this, let's say that my opponent lost 12 lives, now it costs me less 12 manas colorless creature spells, ok and I have my commander Ramos, Dragon Engine, which cost me 12 mana cast since the ah was 4 times in the command zone, but I want to enter with 4 counters on the land of Opal Palace, my question is can I use the 2 skill of Opal palace ?, since with the effect of Rakdos it costs me now 12 less to download it say we do not pay anything to download it, can you tell me if you can use opal palace so that Ramos enters with 4 counters + 1 / + 1
Borrowing the example I gave in this thread, with the additional assumptions that your commander is Ramos, Ramos is in the command zone, you control Rakdos, and your opponents have lost 12 life total this turn:
In this case, Ramos costs an additional 6 (C.R. 903.8), making the cost 12, and then reduced by 12, resulting in a total cost of 0 (C.R. 601.2f). Although you can still activate Opal Palace's second ability (but only before you announce Ramos, since the total cost no longer "includes a mana payment" [C.R. 601.2g]), you can't spend the mana to cast your commander in this case, since there is no mana you're required to pay to do so here (C.R. 601.2h).
ok, then I can activate the Opal palace's ability, so that Ramos enters with counters + 1 / + 1, am I right?
No, you cannot make Ramos enter with additional +1/+1 counters with Opal Place if Rakdos makes it so Ramos' total cost is 0, because you cannot spend mana from Opal Palace to cast it. You cannot spend mana on a cost of 0.
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In this case, Ramos costs an additional 6 (C.R. 903.8), making the cost 12, and then reduced by 12, resulting in a total cost of 0 (C.R. 601.2f). Although you can still activate Opal Palace's second ability (but only before you announce Ramos, since the total cost no longer "includes a mana payment" [C.R. 601.2g]), you can't spend the mana to cast your commander in this case, since there is no mana you're required to pay to do so here (C.R. 601.2h).
EDIT (May 12, 2019): Correctness edit.