<p>We determine the optimal horoball packing densities for Koszul-type Coxeter simplex tilings in hyperbolic 3-space. Using a parametrization of horoballs by the Busemann function and the symmetry of the tilings, we obtain families of packings that attain the universal simplicial density upper bound<Equation ID="Equa"> <EquationSource Format="TEX">\(d_3(\infty) = \big( 2 \sqrt{3} \Lambda\big(\tfrac{\pi}{3}\big) \! \big)^{-1} \approx 0.853276,\)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <msub> <mi>d</mi> <mn>3</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>∞</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mrow> <mo maxsize="1.2em" minsize="1.2em" stretchy="true">(</mo> </mrow> <mn>2</mn> <msqrt> <mn>3</mn> </msqrt> <mi mathvariant="normal">Λ</mi> <mrow> <mo maxsize="1.2em" minsize="1.2em" stretchy="true">(</mo> </mrow> <mstyle displaystyle="false" scriptlevel="0"> <mfrac> <mi>π</mi> <mn>3</mn> </mfrac> </mstyle> <mrow> <mo maxsize="1.2em" minsize="1.2em" stretchy="true">)</mo> </mrow> <mspace width="-0.166667em" /> <msup> <mrow> <mo maxsize="1.2em" minsize="1.2em" stretchy="true">)</mo> </mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mo>≈</mo> <mn>0.853276</mn> <mo>,</mo> </mrow> </math></EquationSource> </Equation>where <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\Lambda\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Λ</mi> </math></EquationSource> </InlineEquation> denotes the Lobachevsky function. These results show that extremal packing densities in <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mathbb{H}^3\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">H</mi> </mrow> <mn>3</mn> </msup> </math></EquationSource> </InlineEquation> are realized by multiple explicit Coxeter tilings and are closely tied to special values of <i>L</i>-functions and hyperbolic manifold volumes.</p>