3. The blood is the life.

sanguis est vita.

"blood" is the subject, "life" the predicate noun after "is." Nominatives all around.

4. Marius drinks the blood of the poet.

Marius bibit sanguinem poetae.

"Marius" is the subject, and nominative. "blood" is the direct object, thus accusative, and "poet" needs to be in the genitive (possession).

5. Marius gave roses to the victim.

Marius dedit rosas victimae. "Victim," the indirect object, needs to be in the dative case.

6. The bright light of the sun kindles my immortal flesh.

Lux splendida solis incendit meam immortalem carnem.

"Light" is the subject, "flesh" the direct object. splendida modifies lux and so must be feminine, singular, and nominative. You need the feminine, singular, accusative forms of meus, -a, -um and immortalis, -e with carnem. "of the sun" is solis, the genitive.

7. House of the Damned (Ones).

Domus Damnatorum.

Use the genitive plural of the adjective damnatus, -a, um. It's conventional in Latin to use the masculine plural if the gender of the people in question is mixed or unspecified.

8. House of the White Rose.

Domus Albae Rosae.

Our friend the genitive again; albus, -a, -um must be genitive singular feminine with the genitive singular of rosa, rosae.

9. Camilla reads the book about the white wolf.

Camilla legit librum de albo lupo.

Here liber, libri is the direct object. The preposition de, "about," must be followed by the ablative case of "wolf," lupo. The adjective modifying "wolf" is thus ablative, singular, and masculine to agree.

10. I am eternal.

Aeternus sum (if you're Marius); aeterna sum (if you're Camilla). Predicate adjectives with linking verbs ("is," etc.) are nominative, just like predicate nouns.

11. My Camilla, come with me into the endless night.

Camilla mea, veni mecum in noctem infinitam.

Technically Camilla is "vocative," the case you use for direct address, but that's identical with the nominative except for second-declension nouns that end in -us, and I'm not going to get into that right now. Mea is feminine, singular, and nominative to agree. veni means "come!", as a command. Mecum means "with me" -- you'll just have to accept that. "Into" is the preposition in followed by the accusative case: noctem. "Endless" must be accusative, singular, and feminine to agree with "night."

12. I gave the poet eternal life.

Dedi poetae vitam aeternam.

"Poet" is the indirect object (dative case), "life" the direct object (accusative case). As always, the adjective must agree with its noun in case, gender, and number, and so, with vitam, is accusative, feminine, and singular.

If you have questions, or if you notice an error on my part, let me know.