It’s pronounced “Seleucus.”
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In practice, you will hear either the Latinized pronunciation (Sell-oo-kus) or that Greek (sell-ehoo-kos). A soft 'c' (sell-oo-sus) is incorrect in both cases, but a frequent way people who've seen the name written but never pronounced will say it.
Seleucid is thus correctly either Sell-oo-kid or Sell-ehoo-kid (for the fancy folks matching the ευ Greek dipthong) but never Sell-oo-sid.